I feel increasing the size of the party has multiple benefits especially for this game
The main one for me is Story because it seems that many NPC actually has interactions with the world like at the start with zorro you need Lar'zel for that meeting it also allows for more interaction between you and other NPC within your party and that helps grow an attachment to them as well as some comedy moments.
And then there also the added benefit of making the game easier for other new players having more than on healing or tank could be priceless for non-experienced players this could be balanced by decreasing the amount of experience your party gets so more experienced player can get more levels faster.
I feel the party size should be at least 5
Another good feature would be group skill checks so depending on who is in your party can be used their skill in the checks as at the moment it seems to be whoever you have selected gets the check and for me that kinda breaks emersion as most people like to play there main as the main while NPC is supporting I feel for multiplayer this system is fine but for single player not so much so they should have 2 systems one for single player and one for multi.