11 3/4" rotors A-Body
for what it's worth...
if you can lock the brakes on a car (or full brake with anti-lock), the only difference in braking distance at that point is the quality of your tires.
having 10, 12, 15 inch rotors 1, 2, 4 piston calipers or real big ones will not affect your braking distance. it's the tires - how soft, how big of a foot print etc.
the only time bigger stuff comes in handy is during endurance racing where heat up becomes a factor...brake fade. the bigger stuff can dissipate more heat when you do a lot of hard braking over a long period of time.
so having bigger rotors really doesnt help most people. they are also important on trucks where you are towing/hauling lots of weight.
just for reference, nascars use 12.72" rotors on 15" wheels...