4 Pot calipers take 294mm rotors. Assuming your car has the 2 pots off an RS it will have 277mm rotors. Single pot Impreza/lib are smaller again at 160 odd.

So yes, if putting on the 4 pots you want rotors off a 99+ WRX.

What wheels do you have? Might want to check they will clear 4 pots as well.