I really like the general idea of narrative limits on party sizes rather than mechanical ones. I agree with you that having Lae'zel stay at camp polishing your spare boots while the real heroes are out adventuring is probably not the best choice though.
In my ideal world, the limit would be neither 4, nor 6, but rather whatever served the story at that point in time. I think it would be great if in some parts of the game you can only have 2 people in your party, while others let you have 6, while still others have you leading an army of thousands.
There were only two invitations to the ball, so Mal had to pick one crew member to come with him to the shindig. One team had to go to Endor to shut down the energy shield while the others blew up the Death Star. There are narrative reasons in literature and cinema to split up teams and limit party sizes so that not every character in the entire franchise is in every scene. I can't think of any reason that those devices wouldn't work just as well in video games.
Narrative reasons like this would be cool, especially if you could switch back and forth between each group’s independent missions.
