Lots of great feedback, for sure.
1. I am in a 10 team league and a 12 team league. The 12 team league is significantly better. Expanding the player pool is important. 60% might be a touch high, but it does seem like your rosters are low, and the number of teams is also low. The 10 team league is kind of boring, but also has a higher fee for entry ($50) and transaction costs.
2. I favor a 25 man roster, but both leagues I am have C, 1B, 2B, 3B, SS, 3 OF, 1 U and 9 pitchers with 7 bench spots and 2 DL spots. It is kind of heavy on the bench spots in comparison.
3. Re: playoff format, it depends on league philosophy. The 12 teams are split into 3 divisions, with one wild card. The schedule is pretty extreme, with 3 inter-divisional matchups, and either 1 or 2 games against teams outside of the division. The frequency of the matchups versus non-divisional foes is determined by previous year standings. (It really didn't take me long to do this, since I manually came up with the schedule last year, before I discovered this place!) I'm not a fan of byes, so there are two 2 week competitions to determine the champion.