The party size in D&D depends on the quest nothing more. A party of 4 is standard it simply depends on the size of the campaign. It doesn't matter to me either way. Party size of 6 would make an already bad party pathing system an absolute nightmare. Unless Larian make the party pathing work a party of 6 is not a good idea imo.
Honestly if they cannot get it working in the engine in their two previous games I think the chances are low they will fix it in this one. Man chaining and unchaining, wibble here and there....no thanks.
Now that is an argument I can agree with. If they don't fix the crappy movement mechanics, 6 would make the game worse. You are right.