Well, I did some research on Youtube and found videos that lead me to think that 3x3 and 6x6 are both possible.
http://www.youtube.com/watch?v=r7yz5nkNdR8
This is a triple battle where the players obviously pick 3 out of 6 to battle.
http://www.youtube.com/watch?v=OUSHOSN_ge4
This is a...