You can actually see that on the RPO sticker, in your glove box. I believe the standard on SSVs is 17" rotors.
About brakes, mine says:
JD9: BRAKE VAC POWER, 17' DISC/DISC, W/VSES, 7700 LBS
JL1: CONTROL,INTEGRATED TRAILER BRAKE
JL4: CONTROL ACTIVE BRAKE
Ron