No Larian has already confirmed party size is 4. But one dev did suggest that that is an issue that is potentially open to change.
I would petition for the OPTION, at least when playing single player, to increase party size to 6 with the understanding that this will make encounters easier/less challenging.
They stated 4 player multiplayer, I don't believe theres been any word on party size. There are screenshots showing 5 party members
