This probably won't be the most popular opinion here, but I prefer a four-person party over six. This is mostly because I don't want to manage that many characters - both from a mechanical perspective (dealing with gear and inventory, picking spells, etc.) and from a story perspective. I want to focus on a characters. In my ideal world, I wouldn't actually have direct control over the companions - I could interact with them and form a plan together, but how they execute that plan (and whether they even stick to the plan) would be entirely up to them and their personality. I like playing from the perspective of my character - not an entire party.
In addition, I rather enjoy having a party that isn't well-rounded. If the tools we have available are restricted, we have to get more creative in the ways that we solve problems - this is a good thing (assuming the game is structured in a way that is sufficiently flexible). If I can have access to everything in one party, that removes some of the more interesting choices I have to make. I'm all for burning bridges and jettisoning companions after act 1 - this dramatically improves replayability.
I agree. Well except for not controlling my party members, personally I like that. All we can do now is just wait and see what Larian decides. There are both sides of the argument in regards to party size, some seem to like it as it is, some seem to want more. I think MAYBE Larian might do 5, but considering their history of 4 party members, I think more than likely it will stay at 4. But only time will tell.