I can't decide between 3, 4, or 5 characters in the party. There are great examples of all three.
For 3: Secret of Mana - melee, defensive magic, offensive magic worked great. Was great in FF7/8/etc. as well, imo, and Chronotrigger.
Four is generic, I guess. Lufia 2 specifically was great with the setup. Main couple, and two jokers (til Dekar gets replaced with Artea, who was dull).
Five (maybe six) is prefect in dungeon crawlers and SRPGs - varied enough to not be boring, and not to much so many that you're always in menus.
I guess one or two main characters, and two support characters (peanut gallery) would be my choice.
Edit: I like benched XP for unused characters. Not full distribution, but like 50%. That way, you won't have a completely worthless character. But at the same time, you won't bench characters that are too weak to do anything at first, and bust them out only in the final boss. *splashes around like a Magikarp*
Edit 2: What I do hate is uneven distribution in the main party. SRPGs a little less so, but why should the healers lag cause they heal? FFTA made leveling healers easy though. They can heal a high level character and get a level in 5 turns, guaranteed.