Yup, I also agree with everyone who's saying the whole "rebalancing" argument is B.S. At the very least Larian can just "balance" the game for a party of 4, even while providing the option of increasing (or decreasing) party size, and nobody would have a problem with that.
I've posted this probably 3 times so far in this megathread, but exp has yet to be mentioned in the current iteration of this discussion.
There is no need to rebalance encounters for different party sizes. A simple fix is to just divide total exp between all the characters in the party. 5- and 6-person parties level up more slowly than 4-person parties, and <4-person parties level up faster. Boom! The game now auto-balances itself and Larian only has to care about balancing for a 4-person party.
If Larian wants to get a bit more complicated, they could slightly adjust the formula for granting exp to different party sizes. E.g., character exp = total exp/(party size+1) or = total exp/(party size)^0.5. But this is a MUCH simpler fix than adjusting every single encounter.
As for companions you don't bring with you (assuming they're not all unceremoniously killed at the end of Act 1), use the DAO solution of auto-leveling them to you main character's level minus 1.