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?
So right from the beginning, the argument about party size and balance is out the window because there is no balance based on party size. Besides, the imps that you first fight, three of them, mind you, are supposed to be much tougher with resistance and so forth, so immediately the first encounter itself is built, actually, with a five or six character party in mind (you, three players and Lae'zel), but it's nerfed because it's too hard for the player if there is only one player and Lae'zel. Three imps with proper stats would most likely wipe the floor with 2 level 1 characters.
And the whole game is like this. Take the wood woads and mud mephits. Nerfed. Why? Too hard for even 4 level 4 characters. You really should have 6 if you used proper stats. So instead of giving us 6, Larian has limited us to 4 and nerfed enemies.
Take the Phase Spider Matriarch fight. It started out several patches ago as such an incredibly difficult fight that it took me a million reloads to do. Why? Because they did some sort of funky homebrews with their spiders and made them impossibly tough for a party of 4 level 4 characters. Now what have they done? Nerfed them so they aren't so tough.
But look at the gith! That fight is still really tough for 4 level 4's, but it wouldn't be so bad for 6 level 4's or even level 5's.
What's my point? The argument about balance is, in my opinion, invalid because they need to work on balance anyway, and they said some time ago that eventually there would be difficulty settings. So, they're going to have to revamp their encounters regardless of party size anyway.
For me, the bottom line is, what makes the most sense for this game? It all goes back to these things:
1. With a party of 6, you can have more characters in your party so you don't have to constantly switch characters out in order to trigger origin character story elements. I can have all the current origin characters in my single player party and not have to switch out one in order to get Shadowheart's full story, or switch out another to get Wyll's or switch out another to get Lae'zel's. All of them can just be in my party at once and we can trigger many more cutscenes without annoyingly having to switch characters out.
2. For 4-player multiplayer, I can still trigger origin story content by having at least 2 origin characters in the party. Thus, multiplayer is more enjoyable, and you can still experience the whole game the way D&D was meant to be played, with others. Right now, that's not possible. You cannot even have Lae'zel interrogate Zorru to find the githyanki patrol because your party is full up with 4.
3. A party of 6 allows players more variety for strategy and combat.
