Because of how the character development system is setup, you can't really min max in the traditional sense. I went with a party of three because it makes your choices matter more. I always found in games where you have 6 party members I had a few worthless characters that were there, but not really substantial, and I always ended up with 1 powerhouse character that didn't really need the rest of the group.
With three each decision has a heavier impact and all three characters feel like powerhouses throughout the game, relying on each other and specialized in multiple roles. Also because of how characters develop in the game, each character "should" develop at a constant rate in relation to the other characters.