The only justification Larian gave for 4 party size that makes sense was to prevent TB combat dragging on too much.
This does seem to be the only reason they have given, which I really don't agree with. When playing SP, you're controlling all of your characters, so there is no real down time except when waiting for AI to move (which do so rather quickly). For MP, more players could become an issue, but with the proposed inclusion of simultaneous turns (if I understand it correctly), a 6 person MP session will likely be even faster than a current 4 person MP session in DOS2. If the pacing is still too slow, a choice of less than 6 party members is still an option (which comes with increased difficulty, but that isn't necessarily bad either).
A consideration against a six person party is the possibility of needing to enlarge the environment to accommodate all the extra bodies, since it is possible that it can get a bit cluttered (but I have my doubts about this too). XP shouldn't be a problem right, since the total XP value of the mobs is divided among the party? If not (i.e. each character gets a set amount of XP for each mob killed), that would require some balancing to avoid leveling too rapidly.
