I'd prefer 8-10 for party size (if there simply MUST be a limit at all)... but I'll certainly +1 the 6 member party idea. 4 is just too limiting in entirely too many ways, especially when you have NPCs that are programed to que off of each other and make certain comments when this person or that person is with the group.
Please list ANY game that offers 8-10 party members? OR even 6? I mean every RPG I can think of at the most has 4. Your not leading a posse...If having a party that size is so important, there is mods for it in every RPG I can think of. But in the last 10 years, I don't think there are any games that go above the 4 party limit. There is also console limitations they have to consider.