I think 6 is too much...4 is enough.
Can you explain why you (as.other) think it's not good to have more choices ? 6 slots is not supposed to mean you HAVE to manage 6 characters if you don't want to.
So what can be the problem ?
Well it does mean that, if the game is balanced and structured for a 6-man party, then your meant to play with a party of 6. (Although I would agree that you do not have to, but you might not doing so problematic)
They would have to include some sort of scaling of enemies or player character strength to allow you to play with less than the default party limit effectively. Technically possible but not necessarily easy.
