I played in a local 8-man league here in Richmond last year, and we played according to the USFFA rulebook:
http://www.usffa.org/rulebook.htm
I really liked the way the USFFA game is played.
I would think we could do 8-on-8, and perhaps even have offensive and defensive squads, if each team can get 16 people. That way, you've got plenty of subs.
Here's some other good sites for flag football guidelines:
http://www.flagfootball.org/StylesOfPlay.htm
http://www.aftfl.com/rulebook.cfm
http://www.nvffa.com/rules04.htm