Of course, with Mamiya RB67, you have a choice.
You certainly do. Mamiya sold film backs for the system in 6x4.5 as well as 6x7 and 6x8 (not sure I see much if any advantage to the latter), and any 2x3 Graflok back will fit, so you can shoot 6x6 as well if you choose. I can well see why this was the system of choice for so many professionals when it was current.


and then again all those 8x10 film holders from playboy went for a fortune.