I like 5 as a size. 4 is easier to schedule and move the action along. 6 was a nightmare of drudgery.
The campaign that I run has 5: fighter/holy liberator, druid, fighter/wizard, cleric/contemplative, ranger/rogue. Insane spellcasting group. Insane melee group. They are just wicked powerful, and well balanced.
The campaign in which I play has 4, and after many PC deaths, and much player turnover, its current state is: barbarian, warmage, cleric, monk/rogue. I think we're well balanced, though we feel a slight pinch at the limits of the warmage's repertoire. The monk/rogue has a ridiculously high Use Magic Device modifier, but that often keeps him from throwing punches. This keeps us kinda low on the melee side.
Ironically, it is easier to schedule the first group than the second.