I think 12x12 is a good size - if you make the board too small the skill involved decreases and the board layout (luck) plays a bigger part in it.

Number of moves would naturally be higher in longer (frequently a larger number of moves) games, and started and completed games will naturally be lower for longer games (for example, backgammon and Spider Line4 are shorter games, and checkers and froglet are longer games). I would say the number of players with either a provisional or established rating (or a certain number of completed games) - that's what they use on GoldToken.