I wonder if its possible to have the game auto-balance to 6 party size - or if they would have to go back and re-balance all the encounters for the larger group.
Probably, and in theory not even that hard. Make a couple of NPC scale in limited level range (i.e. the brigand leader can go from level 6 to level 8), add a couple of random mooks to a fight, etc.
But it's not really that important. The game being "super tight" with its balance is not really something worth fussing over. Especially since it won't be anyway, according to whoever you'll ask.
And "rebalancing everything" really reeks of false concern to begin with, when hardly anything is set in stone yet.
It's also somewhat stupid to worry about it while ignoring that Larian always had multiple difficulty settings in most of their games. Which makes vaguely perplexing when people pretend concern about "all the extra work" this could require.
Why a "Story mode for wimps" and a "super-hardcore masochist mode" are supposed to be a fine use of their time but addressing the request to have an extend party would be the worst waste of their life?