Larger rear rotor diameter will not of itself create an issue, aside from causing more unsprung weigh and possible clearance problems. It will generally cause the rear brakes to run cooler than a smaller rotor due to it's ability to dissipate heat faster.
Rear caliper piston diameter will. The rear caliper piston bore diameter needs to be compatible to the front, and is generally much smaller than the front, because weight transfer under braking reduces the percentage rear brake needed. A larger rear rotor would require an even smaller caliper piston bore because the farther from the center of the rotor it is, the more efficient at slowing the rotor down it is.
Screw relative caliper piston bores up and even an added proportioning valve will not save you, adjustable or not.
Example of a larger rear rotor from the factory is the Mark VII Lincoln (same setup as SVO and Fox body Saleen Mustangs). Probably done to save the cost of new pieces in front when the rear discs from another application were added to the rear of an originally drum braked car, it used 10.9" diameter front rotors with 73MM diameter front piston bores and 11.25" diameter rear rotors with 54MM rear piston bores.
That said, if you're determined to run 12 inch rear rotors, the best plan for the fronts would be one of the AR Engineering kits for using MB 13" front rotors and Viper calipers.
You can't even have too much brake, but you can damn sure have too little.
