Random Brake Rotor Sizing??
 

Ok, so I just got an A6 w/ 2.7t. I've gotta get new brake rotors for it, but I'm curious as to what determines the size of the rotors. When I look up the parts, my searches have come back with three sizes! How did Audi decide when to put the sizes on? Was it random, or is there some other equipment or package that dictates the size of the rotor?