The highest possible dimensions you can use is 8x8 without breaking it. I'm not sure how to check if a puzzle is possible without guessing, so some puzzles may require guesses to solve :(. Press space/click the button in the top left to switch between uncover and mark mode. Uncover mode uncovers tiles you click. If it's a green tile, you're safe, but if it is red, you lose. In mark mode, you can mark tiles you think are red, putting an X over it. the number let you know where the green tiles are. Look up the picross rules if you don't know how to play picross.
