CFUD seems like LJ-based IRC, but IIRC the only requirement is that you post or comment once every two weeks. No activity for a month, and your character is retired. So I guess the ideal number of players would be dependent on how busy it was.

I spent some time poking about the Web for ideas but so far the games with the most info seem to be highly-structured ones like tabula_rasa and fandom_high, which are far less cracktastic than I'm expecting this to be. milliways is another fairly free-form one, I think, but not much info on how it was set up at the beginning.