I really hope the party size increases to 6. It's a Baldur's Gate game after all, and 4 is just a very tiny party. Especially since I like to experience all the companion's stories.
And now I'm seeing some insanity about locking the party after the first act? That's so crazy it almost sounds made-up. If they really are locking the party, then only a party of 4 is very sad, and makes playing an Origin almost mandatory to get a full story experience out of the game since custom PCs have no content.