7 × 7 is the everyday Hitori size: enough room for several duplicate groups per line, so the shading cascade — shade a cell, circle its neighbors, eliminate their twins — starts doing real work.
Expect your first genuine connectivity reasoning here too: a shade that looks legal by the number rules can still be wrong because it would wall off a corner of white cells.
If a board stalls, the how-to-play guide covers the full technique ladder, from pairs and sandwiches up to wall arguments.