Encounters don't have to be rebalanced if you have properly shared XP. A party of 6 will be lower-level than a party of 4, and thus will have the same effective power. A party of one or two will be higher-level than a party of 4, and thus have the same effective power.

Larian would only need to tweak one thing: the exp equation. Which is ~trivial.

Or as kanisatha said, nothing could be changed and a party size of 6 could be explicitly labeled as an "optional and unbalanced" setting. Personally I would never use that option because the game would get too easy=boring, and I'd lose some respect for Larian for not even trying to balance this game mode. But as long as ^ requires a checkbox in settings to enact, I wouldn't be any less happy with the game. And modders could adjust exp gain.