so you want to know how big the rear rotors are on the 6? in case they are the same size as rear on the msp? so you could get mazda 6 rotors/calipers to put on your msp since there arent a lot of options for calipers for the rear of the protege?
You might want to contact Stretch/Steve. He is on these boards, but he's an admin on mazda6tech.com and I think that he might have switched to bigger calipers. He's in MD though.