12 or 14 teams, I actually prefer 14 teams. Scheduling-wise, it works perfectly. Everyone plays everyone else once during the season, and there is a 3-week, 8 team, playoff. You have to do your research to win, but you don't have to have scrubs on your roster.
I actually don't really care what size league it is, just so every manager is very active and competitive. Larger leagues are just harder to win because there are more teams. (8 teams, 1 out of 8 wins; 14 teams, 1 out of 14 wins)