The early FF's traditionally had parties of 4 (with FFIV being an exception), but in the Sony incarnations, have leaned towards commanding a trio of characters (again, FFIX being an exception).
It hasn't taken quite as uniform an approach to the total number of characters in the group, though. Although I,III and V all stuck with 4 selectable characters during the game, other games have ranged from 6 to 14 total party members.
I like the job system, which works most effectively with a larger party. So a party of 4 or 5 would suit me best. I also like my team to level up at a similar pace, so I'd want a group of 8/12 for a party of 4, and 10 for a party of 5.
Of course, this system would only work without the hindrance of a permanent main character, which really screws up the levelling process. If you want a good example, look at FFVIII.
How many characters do you like in your party, and how many in the group as a whole?