It really depends on whose rules you run under, a 17.5 ROAR legal has certain rotor specs, but that doesn't mean they don't get changed out at the club level. Same for some of the 13.5 spec motors, but when nothing is teched and a simple look for a tag, like the old days, means there could be quite a bit of rotor changing going on.
Although you can have variables such as outside diameter of the rotor, there are other things that can also change the characteristics of your motor. The actual inner shaft diameter will determine the thickness of you rotor, which again will effect perfomance. Another lesser explored variable is the actual length of the rotor as well, but thats another can of worms. Orion offers a rotor tuning kit with 4 different rotors to tune your brushless mod motor with, but there is nothing stopping you from using that in a other application.