10 × 10 is the classic Shikaku size — the one Nikoli made famous. A hundred cells split into fifteen to thirty rectangles means you can't brute-force your way through; you need a scanning rhythm. Strong solvers sweep for forced strips first (primes and edge-pinned clues), then work the contested middle where large clues overlap.
Expect areas up to 20 on our hard boards, with clues placed to keep three or four candidate rectangles alive at once. When you stall, look for orphans: an empty cell that only one remaining rectangle shape can reach is as forcing as any clue.
If 10 × 10 starts feeling comfortable, 15 × 15 is the next step up — or test yourself against the hard tier at this size first.