Weirdly I find the 15x15 puzzles to be easier in general than the 6x6 ones. Maybe it is because the 15x15 ones aren't packed as tightly so you don't have to us

Weirdly I find the 15x15 puzzles to be easier in general than the 6x6 ones. Maybe it is because the 15x15 ones aren't packed as tightly so you don't have to use as much second order logic? The only suggestion I would make is have a middle click that marks a spot as a possibility.