20 rooms on a side
Twenty rooms is Expert when keys lock the exit. Three diamonds sit in dead ends.
Classic twenty is a long walk with many stubs. Keys mode plants three diamonds; standing on the coral gate early does nothing. Expect several minutes if you clear every branch.
The number is rooms, not pixels. Walls sit between rooms, so the drawing is 41 tiles across. Start is top-left; exit is bottom-right. Same seed, size, and mode rebuild the same corridors.
How to walk it
Arrows and swipe take a whole corridor and stop at the next fork. Yellow marks where you moved — not a solved path. Hint draws one solid arrow; Peek flashes the shortest route. Either leaves that run off local Marks.
Also: all room counts, make desk, print this size, right-hand rule.