You can't base party size on encounter balance. Why? Because party size is already too varied. From the very beginning of the game, the first encounter, you could have the MC, Lae'zel and Us OR just you and Lae'zel OR you, three other players, Lae'zel and Us, or you, three other players and Lae'zel. It depends on who you have playing. Is it single-player mode or multi?
This is it exactly! Party size is already something that can vary within the game, by Larian's own design! So arguing against the option of a party size of six based on encounter design is obviously an invalid argument.
Wrong, it actually a proof of how party size affects difficulty in encounters. Killing commander Zhalk with just Lae'zel is extremely difficult, with Lae'zel, US and Shadowheart it's a piece of cake.
No, you're the one who's wrong. And in being wrong you are making my argument for me by admitting the game already can be played with varying party sizes from 1 to 4. So if the game allows us to play with party size 1, as you yourself say here that it does, AND not a single person anywhere is demanding that the game should be balanced for party size 1, then why not also allow party sizes of 5 or 6 in exactly that same way? There is zero difference between the game allowing us to play with party size 1 versus party size 6, even while being balanced for party size 4.