That's easy. A party of 6 would make rework of the encounters necessary, despite what friends of the party of 6 say.
No, a rework of encounters isn't necessary.
As @Rag says above, the absolute minimum Larian could do is simply allow an OPTIONAL max party size of 6 without making ANY additional changes. For people who play with a party of 4, the game is exactly the same. For people who play with a party of 6, either their game is easier or they increase the game difficulty (BG3 will almost certainly include difficulty options) to compensate.
If Larian wants to do slightly more work, a relatively easy balancing is to adjust exp gain for differently-sized parties. A party of 6 gains less exp - and thus levels up more slowly - than a party of 4, which would work to balance combats. E.g., 4 level-3 characters vs 6 level-2 characters are roughly similar in power.
+1 Most games have difficulty settings. Even in DOS2 IIRC. If there are custom settings that would even be better. I'm sure there probably be modders to "enhance" the combat encounters hence i believe a max party of 6 would be really enjoyable. Those who wanted 4 they can still play the way they want. Everyone wins.