Ideal Party Size

I remember seeing a thread similar to this recently, but I can't find it now. Anyway: What would be the ideal party size for getting through the game easily / not too slowly? Once OB comes around, I plan on playing with my 3 brothers. Would 4 people in the same party be an alright size, or would it be better to drop to two groups of two? Not looking to rush through the game, but also don't want to get through it at a snail's pace. I'll probably end up soloing later on anyway, but was still curious.

How many ppl can be in a party right now?

6 is the max size

Call me old fashioned, but I can't help but wonder if 5 party members would be ideal. If there are going to be 6 classes, you shouldn't be able to form a party covering every class. This would leave you with some choices to make about how you build a party imo.

Should be 1-2 above the class size.