Wheel size for 12" disc brakes?

[77GMCTruck]

I'm trying to find a site or info on how big the wheels have to be for a 12" disc brake kit? I found a company that makes IFS and 4 link rear suspensions for our year model trucks (60-87) and they offer a Wilwood or Baer Brake system 12" or 13" setup. Of course this is a long ways down the road for me, and I can't afford the 13" discs, but I was wondering if anyone knew or knew of a site for calculating it?

My brother bought a new Supercharged Shelby GT and he's got a full set of Torque Thrust 2 17" Shelby Rims that he's talking about giving me for the truck. His are black but he's going to talk to his friends at Shelby Automotive about swapping them (never been used) for the Anthracite ones.

Anyways, if I could save a grand on rims it would give me more to play with in other areas.

Thanks in advance for your help.