Bigger IS Better
I didn't want to replace the wheels on my 1988 Fiero GT (some brake upgrades require 16" or larger rims), so the first thing I had to do was determine how large a rotor I could fit under the stock 15" wheels. My car has four identical rotors, so I removed one wheel and one rotor and set them up on the bench with a spare caliper I had laying around. I aligned the caliper so the pads matched the wear pattern on the rotor. I could see that there was quite a bit of room between the top of the caliper and the inside of the wheel. I slid the caliper toward the wheel until it hit, and measured the distance between the bottom of the pad (this would be the new Ri) and the bottom of the wear pattern (old Ri). For my car, this dimension is about 3/4", which means I can improve the moment arm of the calipers by 3/4". Then I multiplied the old Ro by two and added 1.5" to determine the new diameter of the as-yet-found larger rotor. This is the largest rotor I can use with the stock wheels.
After that, I measured all the dimensions of the old rotor, i.e. bolt pattern, offset, etc., so that I could match it with a larger rotor.
Finding a larger rotor that fit my car proved to be a challange. Most places I called could not give me actual dimensions other than the diameter and bolt pattern. The other dimensions are probably even more important. I knew that I'd have to drill the rotors to match the hole pattern, and probably the new rotor would have a larger hub size. That means I'd have to make a ring to fit between the hub on the car and the center hole of the rotor. The other important dimension is the thickness of the rotor and the 'hat' or offset dimension. I had to have a close match with the thickness in order to use the original calipers, and I needed at least as much offset as the original rotors. This is because I would need to make adapter plates to mount the calipers 3/4" further out on the radius, so I would need some space for the thickness of the plates.
I took dimensions of the car where the calipers bolted on. Then I drew a layout in AutoCAD and used that as my design model. In ACAD, I 'moved' the calipers to the new position and found that I would have interference between the bolt heads and the caliper, so in addition to translating the calipers out, I also had to rotate them about the center of the rotor. This gave me enough clearance for the heads of the mounting bolts.
Even so, the brackets were very complex and difficult to make. I used cold rolled steel for the bracket material because threads are required to mount the calipers, and I didn't want to rely on aluminum. I suppose I could have used threaded inserts, but these things were tough enough to machine without having to align inserts as well.