15 × 15 is the marathon format — 225 cells, values 1 through 15, and duplicate structures big enough that a single opening read can take a few minutes on its own.
Solving one cleanly is mostly discipline: work the certain deductions first (triples, pairs, sandwiches), circle everything a shade proves white, and only then reach for connectivity arguments. The cascade does the rest.
These boards assume you know the full ladder — the how-to-play guide is the reference, and the 12 × 12 boards are the right training ground.