There is more to resisting brake fad than simply going to larger rotors. Since the brake is a heat sink, you can increase the mass of the rotor to better disapate heat. 1.25 thick vs 1.0 makes a difference. There also is the ducting of air into the rotor to remove heat. You wouldn't expect a radiator to work effectively with no airflow, brake rotors work the same way. There are also pad compounds to consider. This is one of the reason to go to an aftermarket caliper like Wilwood or Viper. The variety of pad compounds allow you to tailer the heat range they are effective in. Compared to teh stock pad selection, this is the area to make your biggest gains.

FWIW, in autocross you actually can get away with a lot smaller and lighter rotor because the braking forces will be occurring at slowers speeds for less duration. Also consider that the nascar boys have been using 12.19