The maximum wire size depends on the size of lug you are using. The clearance for the #6 output terminal screw is .297 " and our largest lug within that tolerance will accept #12 -#10 wire size.
Lug options:
06122-06 #12- #10 10 pk
06122-01 #16- #14 10 pk
06122-02 #22- #18 10 pk