Why did BG1 chose to have 6 chars? Since then lots of people think every RPG must have a party of 6. ( see this thread ).
As far as I know, PnP DnD has been designed for 4 players plus 1 DM.
I have played several computer RPGs with different party sizes. I liked most of them and some were bad, but the party size was never the main factor why I liked or disliked a game. The main problem with party size were games with way more party members than active party members and the ones who are not in the party do not gain exp. This means grinding for dungeons were you need several different active groups or when you need a specific char for one area or quest. I never finished Final Fantasy 6 because I hated grinding when some late game dungeons require several active parties at once.
Its not necessarily true that Baldurs Gate set some kind of standard for the genre when it comes to party size but you make a good point about PnP becoming to bloated with too many players. And since a key feature (I think it is) of BG3 is the coop mode adding 2 extra players it might not work that well. In a single player game you are the star and every other character is essentially disposable and party siize wouldnt matter except for balancing issues.
I'm curious to see statistics from the D:OS games of exactly how many people play coop or single player, or both.