i have been in a 16 team keeper league for about 6 years now. and i love it!!! i consider myself a knowledgeable fantasy footballer therefore i usually do pretty good. you have to know your stuff, i will tell you that.
last year, the biggest moron in the league won it because he had j.lewis then picked up boldin off waivers and aquired moss in a trade from a team that was dumping guys because that team was out of it already.
i don't think it is that big of a deal. if we had 20 guys, i think it would still be fun. you just have to know almost all the guys in the league. of course you are going to have spots where your team is weak but that is the fun of it. i find 8 team leagues stupid. every team has a stud at every position. that isn't fun to me at all. there is no managing involved when you play the same studs every week. with more teams, there is more manageing.
go with a regular 20 team league and just make sure all the guys are prepared for the amount of guys that will be drafted. it will be a blast. break it up into divisions and put friends/enemies in the same division to make it more fun.