diff --git a/app/features/tournament-bracket/components/Bracket/Bracket.browser.test.tsx b/app/features/tournament-bracket/components/Bracket/Bracket.browser.test.tsx
index 1c4e0a710..8c72f8a20 100644
--- a/app/features/tournament-bracket/components/Bracket/Bracket.browser.test.tsx
+++ b/app/features/tournament-bracket/components/Bracket/Bracket.browser.test.tsx
@@ -117,7 +117,7 @@ function createSingleEliminationData(): TournamentManagerDataSet {
 				number: 1,
 				type: "single_elimination",
 				tournament_id: 1,
-				settings: { size: 8 },
+				settings: {},
 			},
 		],
 		group: [{ id: 1, number: 1, stage_id: 1 }],
@@ -228,7 +228,7 @@ function createByeHeavySingleEliminationData(): TournamentManagerDataSet {
 				number: 1,
 				type: "single_elimination",
 				tournament_id: 1,
-				settings: { size: 8 },
+				settings: {},
 			},
 		],
 		group: [{ id: 1, number: 1, stage_id: 1 }],
@@ -342,7 +342,7 @@ function createDoubleEliminationData(): TournamentManagerDataSet {
 				number: 1,
 				type: "double_elimination",
 				tournament_id: 1,
-				settings: { size: 4 },
+				settings: {},
 			},
 		],
 		group: [
@@ -443,7 +443,7 @@ function createRoundRobinData(): TournamentManagerDataSet {
 				number: 1,
 				type: "round_robin",
 				tournament_id: 1,
-				settings: { groupCount: 2, size: 6 },
+				settings: { groupCount: 2 },
 			},
 		],
 		group: [
@@ -673,7 +673,7 @@ function createLargeSingleEliminationData(options?: {
 				number: 1,
 				type: "single_elimination",
 				tournament_id: 1,
-				settings: { size: 16 },
+				settings: {},
 			},
 		],
 		group: [{ id: 1, number: 1, stage_id: 1 }],
diff --git a/app/features/tournament-bracket/core/engine/create/builder.ts b/app/features/tournament-bracket/core/engine/create/builder.ts
index 4770e9415..6280a0ef5 100644
--- a/app/features/tournament-bracket/core/engine/create/builder.ts
+++ b/app/features/tournament-bracket/core/engine/create/builder.ts
@@ -29,24 +29,22 @@ import {
 export class StageCreator {
 	readonly input: ResolvedCreateBracketInput;
 	settings: StageSettings;
-	seeding: Seeding | undefined;
+	seeding: Seeding;
 	readonly data: BracketData;
 
 	constructor(input: ResolvedCreateBracketInput) {
-		this.input = input;
-		this.settings = structuredClone(input.settings) ?? {};
-		const seeding = input.seeding ? [...input.seeding] : undefined;
-		this.seeding =
-			seeding && input.type !== "round_robin"
-				? padSeedingToPowerOfTwo(seeding)
-				: seeding;
-		this.data = { stage: [], group: [], round: [], match: [] };
-
 		if (!input.name) throw Error("You must provide a name for the stage.");
 
 		if (!Number.isInteger(input.tournamentId))
 			throw Error("You must provide a tournament id for the stage.");
 
+		this.input = input;
+		this.settings = structuredClone(input.settings) ?? {};
+		const seeding = [...input.seeding];
+		this.seeding =
+			input.type !== "round_robin" ? padSeedingToPowerOfTwo(seeding) : seeding;
+		this.data = { stage: [], group: [], round: [], match: [] };
+
 		if (input.type === "single_elimination")
 			this.settings.consolationFinal = this.settings.consolationFinal || false;
 	}
@@ -161,7 +159,7 @@ export class StageCreator {
 		number: number,
 		losers: ParticipantSlot[][],
 	): ParticipantSlot {
-		const participantCount = this.settings.size!;
+		const participantCount = this.seeding.length;
 		const roundPairCount = helpers.getRoundPairCount(participantCount);
 
 		let losersId = 0;
@@ -330,31 +328,13 @@ export class StageCreator {
 	}
 
 	/**
-	 * Returns a list of slots.
-	 * - If `seeding` was given, uses it.
-	 * - If `size` was given, only returns a list of empty slots.
+	 * Returns the list of slots from the seeding.
 	 *
 	 * @param positions An optional list of positions (seeds) for a manual ordering.
 	 */
 	getSlots(positions?: number[]): ParticipantSlot[] {
-		const size = this.settings.size || this.seeding?.length || 0;
-		helpers.ensureValidSize(this.input.type, size);
-
-		if (size && !this.seeding)
-			return Array.from(Array(size), (_: ParticipantSlot, i) => ({
-				id: null,
-				position: i + 1,
-			}));
-
-		if (!this.seeding) throw Error("Either size or seeding must be given.");
-
-		this.settings = {
-			...this.settings,
-			size, // Always set the size.
-		};
-
+		helpers.ensureValidSize(this.input.type, this.seeding.length);
 		helpers.ensureNoDuplicates(this.seeding);
-		this.seeding = helpers.fixSeeding(this.seeding, size);
 
 		return this.getSlotsUsingIds(this.seeding, positions);
 	}
diff --git a/app/features/tournament-bracket/core/engine/create/double-elimination.test.ts b/app/features/tournament-bracket/core/engine/create/double-elimination.test.ts
index dd61070b6..ddaf1833e 100644
--- a/app/features/tournament-bracket/core/engine/create/double-elimination.test.ts
+++ b/app/features/tournament-bracket/core/engine/create/double-elimination.test.ts
@@ -31,7 +31,7 @@ describe("Create double elimination stage", () => {
 			name: "Example with 256 participants",
 			tournamentId: 0,
 			type: "double_elimination",
-			settings: { size: 256 },
+			seeding: Array.from({ length: 256 }, (_, i) => i + 1),
 		});
 	});
 
diff --git a/app/features/tournament-bracket/core/engine/create/double-elimination.ts b/app/features/tournament-bracket/core/engine/create/double-elimination.ts
index 531917320..37d44d9bd 100644
--- a/app/features/tournament-bracket/core/engine/create/double-elimination.ts
+++ b/app/features/tournament-bracket/core/engine/create/double-elimination.ts
@@ -21,7 +21,7 @@ export function createDoubleElimination(creator: StageCreator): void {
 		ordered,
 	);
 
-	if (helpers.isDoubleEliminationNecessary(creator.settings.size!)) {
+	if (helpers.isDoubleEliminationNecessary(slots.length)) {
 		const winnerLb = creator.createLowerBracket(stage.id, 2, losersWb);
 		createGrandFinal(creator, stage.id, winnerWb, winnerLb);
 	}
diff --git a/app/features/tournament-bracket/core/engine/create/index.ts b/app/features/tournament-bracket/core/engine/create/index.ts
index 24a533131..a929b4d00 100644
--- a/app/features/tournament-bracket/core/engine/create/index.ts
+++ b/app/features/tournament-bracket/core/engine/create/index.ts
@@ -31,7 +31,7 @@ export function create(input: CreateBracketInput): CreatedBracket {
 /**
  * Engine-internal `create` taking already-resolved internal stage settings.
  * Tests use this to control knobs that are an implementation detail to the
- * app (seed ordering, byes balancing, TBD slots via `settings.size`).
+ * app (seed ordering, byes balancing).
  */
 export function createResolved(
 	input: ResolvedCreateBracketInput,
diff --git a/app/features/tournament-bracket/core/engine/create/round-robin.test.ts b/app/features/tournament-bracket/core/engine/create/round-robin.test.ts
index ae9edba2d..1ac9b11ad 100644
--- a/app/features/tournament-bracket/core/engine/create/round-robin.test.ts
+++ b/app/features/tournament-bracket/core/engine/create/round-robin.test.ts
@@ -144,14 +144,14 @@ describe("Create a round-robin stage", () => {
 		expect(realMatchRound.number).toBe(1);
 	});
 
-	test("should create a round-robin stage with to be determined participants", () => {
+	test("should create a round-robin stage split across multiple groups", () => {
 		bracket.create({
 			name: "Example",
 			tournamentId: 0,
 			type: "round_robin",
+			seeding: Array.from({ length: 16 }, (_, i) => i + 1),
 			settings: {
 				groupCount: 4,
-				size: 16,
 			},
 		});
 
@@ -181,6 +181,7 @@ describe("Create a round-robin stage", () => {
 				name: "Example",
 				tournamentId: 0,
 				type: "round_robin",
+				seeding: [1, 2, 3, 4],
 			}),
 		).toThrow("You must specify a group count for round-robin stages.");
 	});
@@ -191,9 +192,9 @@ describe("Create a round-robin stage", () => {
 				name: "Example",
 				tournamentId: 0,
 				type: "round_robin",
+				seeding: [1, 2, 3, 4],
 				settings: {
 					groupCount: 0,
-					size: 4,
 				},
 			}),
 		).toThrow("You must provide a strictly positive group count.");
diff --git a/app/features/tournament-bracket/core/engine/general.test.ts b/app/features/tournament-bracket/core/engine/general.test.ts
index 0dc24a812..a3598227a 100644
--- a/app/features/tournament-bracket/core/engine/general.test.ts
+++ b/app/features/tournament-bracket/core/engine/general.test.ts
@@ -37,10 +37,8 @@ describe("BYE handling", () => {
 			name: "Example with BYEs",
 			tournamentId: 0,
 			type: "double_elimination",
-			seeding: [1, 2],
-			settings: {
-				size: 4,
-			},
+			seeding: [1, 2, null, null],
+			settings: {},
 		});
 
 		expect(bracket.match(0).opponent1?.id).toBe(1);
@@ -59,9 +57,8 @@ describe("Position checks", () => {
 			name: "Example with double grand final",
 			tournamentId: 0,
 			type: "double_elimination",
-			settings: {
-				size: 8,
-			},
+			seeding: [1, 2, 3, 4, 5, 6, 7, 8],
+			settings: {},
 		});
 	});
 
@@ -132,37 +129,22 @@ describe("Special cases", () => {
 		expect(bracket.match(0).opponent2).toBe(null);
 	});
 
-	test("should throw if the size of a stage is not a power of two", () => {
-		expect(() =>
-			bracket.create({
-				name: "Example",
-				tournamentId: 0,
-				type: "single_elimination",
-				settings: { size: 3 },
-			}),
-		).toThrow(
-			"The library only supports a participant count which is a power of two.",
-		);
-	});
-
 	test("should throw if the participant count of a stage is less than two", () => {
 		expect(() =>
 			bracket.create({
 				name: "Example",
 				tournamentId: 0,
 				type: "single_elimination",
-				settings: { size: 0 },
+				seeding: [],
 			}),
-		).toThrow(
-			"Impossible to create an empty stage. If you want an empty seeding, just set the size of the stage.",
-		);
+		).toThrow("Impossible to create a stage with less than 2 participants.");
 
 		expect(() =>
 			bracket.create({
 				name: "Example",
 				tournamentId: 0,
 				type: "single_elimination",
-				settings: { size: 1 },
+				seeding: [1],
 			}),
 		).toThrow("Impossible to create a stage with less than 2 participants.");
 	});
@@ -218,9 +200,7 @@ describe("Reset match", () => {
 			tournamentId: 0,
 			type: "single_elimination",
 			seeding: [1, null, null, null, null, null, null, 2],
-			settings: {
-				size: 8,
-			},
+			settings: {},
 		});
 
 		bracket.updateMatch({
diff --git a/app/features/tournament-bracket/core/engine/helpers.ts b/app/features/tournament-bracket/core/engine/helpers.ts
index 985f736f6..efc68a321 100644
--- a/app/features/tournament-bracket/core/engine/helpers.ts
+++ b/app/features/tournament-bracket/core/engine/helpers.ts
@@ -8,7 +8,6 @@ import type {
 	ParticipantResult,
 	ParticipantSlot,
 	Result,
-	Seeding,
 	SeedOrdering,
 	Side,
 	StageData,
@@ -294,27 +293,6 @@ export function ensureNoDuplicates<T>(array: (T | null)[]): void {
 		throw new Error("The seeding has a duplicate participant.");
 }
 
-/**
- * Fixes the seeding by enlarging it if it's not complete.
- *
- * @param seeding The seeding of the stage.
- * @param participantCount The number of participants in the stage.
- */
-export function fixSeeding(
-	seeding: Seeding,
-	participantCount: number,
-): Seeding {
-	if (seeding.length > participantCount)
-		throw Error(
-			"The seeding has more participants than the size of the stage.",
-		);
-
-	if (seeding.length < participantCount)
-		return Array.from(Array(participantCount), (_, i) => seeding[i] || null);
-
-	return seeding;
-}
-
 /**
  * Ensures that the participant count is valid.
  *
@@ -325,11 +303,6 @@ export function ensureValidSize(
 	stageType: StageType,
 	participantCount: number,
 ): void {
-	if (participantCount === 0)
-		throw Error(
-			"Impossible to create an empty stage. If you want an empty seeding, just set the size of the stage.",
-		);
-
 	if (participantCount < 2)
 		throw Error("Impossible to create a stage with less than 2 participants.");
 
diff --git a/app/features/tournament-bracket/core/engine/propagation/traversal.ts b/app/features/tournament-bracket/core/engine/propagation/traversal.ts
index 50fbc0127..f51a73c55 100644
--- a/app/features/tournament-bracket/core/engine/propagation/traversal.ts
+++ b/app/features/tournament-bracket/core/engine/propagation/traversal.ts
@@ -417,7 +417,7 @@ export class Propagator {
 		roundNumber: number,
 	): MatchData[] {
 		if (stage.type === "single_elimination")
-			return this.getPreviousMatchesFinalSingleElimination(match, stage);
+			return this.getPreviousMatchesFinalSingleElimination(match);
 
 		return this.getPreviousMatchesFinalDoubleElimination(match, roundNumber);
 	}
@@ -426,15 +426,13 @@ export class Propagator {
 	 * Gets the matches leading to the given match, which is in a final group (consolation final).
 	 *
 	 * @param match The current match.
-	 * @param stage The parent stage.
 	 */
 	private getPreviousMatchesFinalSingleElimination(
 		match: MatchData,
-		stage: StageData,
 	): MatchData[] {
 		const upperBracket = this.getUpperBracket(match.stage_id);
 		const upperBracketRoundCount = helpers.getUpperBracketRoundCount(
-			stage.settings.size!,
+			this.participantCount(match.stage_id),
 		);
 
 		const semiFinalsRound = this.store.selectFirst("round", {
@@ -648,7 +646,7 @@ export class Propagator {
 
 		const roundNumberLB = roundNumber > 1 ? (roundNumber - 1) * 2 : 1;
 
-		const participantCount = stage.settings.size!;
+		const participantCount = this.participantCount(match.stage_id);
 		const method = helpers.getLoserOrdering(participantCount, roundNumberLB);
 		const actualMatchNumberLB = helpers.findLoserMatchNumber(
 			participantCount,
@@ -860,6 +858,26 @@ export class Propagator {
 		return winnerBracket;
 	}
 
+	/**
+	 * Gets the participant count of an elimination stage, derived from its upper
+	 * bracket's first round (two participants per match, BYEs included).
+	 *
+	 * @param stageId ID of the stage.
+	 */
+	private participantCount(stageId: number): number {
+		const upperBracket = this.getUpperBracket(stageId);
+		const firstRound = this.store.selectFirst("round", {
+			group_id: upperBracket.id,
+			number: 1,
+		});
+		if (!firstRound) throw Error("First round not found.");
+
+		const firstRoundMatches = this.store.selectAll("match", {
+			round_id: firstRound.id,
+		});
+		return firstRoundMatches.length * 2;
+	}
+
 	/**
 	 * Gets the loser bracket.
 	 *
diff --git a/app/features/tournament-bracket/core/engine/types.ts b/app/features/tournament-bracket/core/engine/types.ts
index 4162c17d1..94d833edd 100644
--- a/app/features/tournament-bracket/core/engine/types.ts
+++ b/app/features/tournament-bracket/core/engine/types.ts
@@ -58,10 +58,6 @@ export type MatchStatus = (typeof MatchStatus)[keyof typeof MatchStatus];
  * TournamentStage.settings today (the old brackets-model StageSettings).
  */
 export interface StageSettings {
-	// xxx: why not inferred?
-	/** The number of participants. */
-	size?: number;
-
 	/** Number of groups in a round-robin stage. */
 	groupCount?: number;
 
@@ -230,15 +226,10 @@ export interface CreateBracketInput {
 
 /**
  * Engine-internal variant of {@link CreateBracketInput}: settings are the
- * already-resolved internal {@link StageSettings} and seeding may be omitted
- * in favor of `settings.size` (TBD slots).
+ * already-resolved internal {@link StageSettings}.
  */
 export interface ResolvedCreateBracketInput
-	extends Omit<
-		CreateBracketInput,
-		"seeding" | "settings" | "independentRounds"
-	> {
-	seeding?: Seeding;
+	extends Omit<CreateBracketInput, "settings" | "independentRounds"> {
 	settings: StageSettings;
 }
 
diff --git a/app/features/tournament-bracket/core/tests/mocks-li.ts b/app/features/tournament-bracket/core/tests/mocks-li.ts
index 329606fd4..0f9b4d0b8 100644
--- a/app/features/tournament-bracket/core/tests/mocks-li.ts
+++ b/app/features/tournament-bracket/core/tests/mocks-li.ts
@@ -22,9 +22,7 @@ export const LOW_INK_DECEMBER_2024 = (): TournamentData => ({
 				id: 1429,
 				name: "Beta Bracket",
 				number: 2,
-				settings: {
-					size: 16,
-				},
+				settings: {},
 				tournament_id: 815,
 				type: "double_elimination",
 				createdAt: 1734286247,
@@ -33,9 +31,7 @@ export const LOW_INK_DECEMBER_2024 = (): TournamentData => ({
 				id: 1430,
 				name: "Alpha Bracket",
 				number: 3,
-				settings: {
-					size: 16,
-				},
+				settings: {},
 				tournament_id: 815,
 				type: "double_elimination",
 				createdAt: 1734286249,
@@ -44,9 +40,7 @@ export const LOW_INK_DECEMBER_2024 = (): TournamentData => ({
 				id: 1431,
 				name: "Gamma Bracket",
 				number: 4,
-				settings: {
-					size: 16,
-				},
+				settings: {},
 				tournament_id: 815,
 				type: "double_elimination",
 				createdAt: 1734286340,
diff --git a/app/features/tournament-bracket/core/tests/mocks-sos.ts b/app/features/tournament-bracket/core/tests/mocks-sos.ts
index b96028151..aa0afc026 100644
--- a/app/features/tournament-bracket/core/tests/mocks-sos.ts
+++ b/app/features/tournament-bracket/core/tests/mocks-sos.ts
@@ -12,7 +12,6 @@ export const SWIM_OR_SINK_167 = (
 				number: 1,
 				settings: {
 					groupCount: 11,
-					size: 44,
 				},
 				tournament_id: 672,
 				type: "round_robin",
diff --git a/app/features/tournament-bracket/core/tests/mocks.ts b/app/features/tournament-bracket/core/tests/mocks.ts
index 25d57b837..6eb9cd861 100644
--- a/app/features/tournament-bracket/core/tests/mocks.ts
+++ b/app/features/tournament-bracket/core/tests/mocks.ts
@@ -10,7 +10,6 @@ export const PADDLING_POOL_257 = () =>
 					number: 1,
 					settings: {
 						groupCount: 9,
-						size: 35,
 					},
 					tournament_id: 27,
 					type: "round_robin",
@@ -6822,7 +6821,6 @@ export const PADDLING_POOL_255 = () =>
 					number: 1,
 					settings: {
 						groupCount: 9,
-						size: 35,
 					},
 					tournament_id: 18,
 					type: "round_robin",
@@ -14210,9 +14208,7 @@ export const IN_THE_ZONE_32 = ({
 					id: 16,
 					name: "Main bracket",
 					number: 1,
-					settings: {
-						size: 32,
-					},
+					settings: {},
 					tournament_id: 11,
 					type: "double_elimination",
 				},