Cobra's had 13" front rotors and dual piston calipers, 11.65" rotors for the rears.
Both sizes are larger than stock GT brakes.
Like said above make sure you get Cobra pads.
you cant just throw cobra calipers on your gt no. reason I was asking to make sure there cobra brakes is it sounds like ur new to the stang world? some people when selling cars say they have cobra this and that when even though they might not (paticuarly dealerships).