RB series camera bodies were designed as 6x7 camera. For this size, you get a 6x7 back and remove any masks in the view finder if any. When you look into your view finder, do you see a cross looking opening? (picture attached) If so, take off your view finder and remove this thing. Now you'll see the whole film area for 6x7.

You can also get 645 back (the whole back, case and the insert) and this mask.

You can also get 6x6 back (the whole back, case and the insert) and I'm not sure about the mask

Or, you can get 6x8 back. This is a power back that winds electro-mechanically. There are two types, one with integral battery and one with an external battery pack. It is supposed to come with a new focusing screen. If you have an older type body, such as Pro or an older Pro-S without "8" on the body behind the rotating back, you'll need to replace what is called a "buffle". Pro-SD does not need any modification.

By the way, that's a heck of a camera for your first film body!