Brake questions, rear caliper piston and rotor diameters please

[Face GS C5]

I have a 99 Regal GS that I upgraded the front calipers to C5 calipers. The piston diameter is less than my Regal's, so my bias is off. I'm looking for a set of calipers that could bolt on in the rear that also use less fluid than stock. Can anyone measure the rear caliper piston diameter?
Also, what's the diameter of the rear rotors?
TIA!