20 × 20 is the marathon: 400 cells, forty-plus rectangles, and no way to hold the whole board in your head. Solving one is a campaign — claim the corners and edges early, let the borders of finished rectangles become new walls, and work inward region by region.
Because every board here is machine-verified to have exactly one solution, you never need to guess, even an hour in. If you're stuck, it's not ambiguity — there is always a forced move somewhere on the board, usually a stranded cell only one rectangle can still reach.
These reward a saved session: your progress is stored automatically, so a giant board can span several sittings. For a shorter daily habit, the daily puzzle stays in the 5–10 range; for a middle ground, try 15 × 15.