I personally really enjoy the party size, played through all of the early access and the difficulty seems just right.
I'm gald for you, you get to play your game how you like best, this thread isn't about taking that option away from you, it's about give people who like the six party member set up from the original BG games, How do you feel about the option to choose your party size be it 1, 4, 5 or 6 so everyone can play how they want?
don't overlook giving feedback on the issue of how the party is managed/moved around, because solving it is basically a pre-requirement to actually get the party expansion you want. The chain/unchain system and its clumsy auto-follow work already poorly enough now. With six men in your party it would turn into an unmitigated disaster.
I'd be happy with selectable formations the same as in the old Infinity engine games, I think it worked really well in them and with a little tweaking could probably service this game quite nicely.