I would prefer larger parties, not for some game mechanic reason, but because I love to have as many party members interacting as much as possible.
I don't give a flying red dragon if there's a change in difficulty, I want to be able to have those interactions go off as much as possible. I love Baldur's Gate for the characters and story not the mediocre ADnD game system, I played NWN, I loved NWN:SoU not for the clanky and painful 3E interpretation, I love NWN2 for their characters not the mediocre 3.5E translation.
I will love BG3 but it is all based on the characters and their interactions, combat can such an egg as long as the companions are deep and interesting. The more interactions they have the happier I will be with the game, so sign me up for 6 party.
Because even with 6 party members O will replay the shit out of this game and I will play it with Minsc every single time, just like BG2, (as long as the rumours are true).