But if they're rear calipers, you'll generally have to push one of the pistons out enough to measure it...there are at least three sizes for the rear pistons.

Just remove one of the pads and the dust boot and use the brake pedal to push out the piston. Remove the caliper and use a vernier caliper to measure the piston.

Why do you think you need new clalipers? 99% of the time the problem is the guide bushings that the front calipers float on. Granted, the rear calipers do fail and hang up. But often they can be simply cleaned up and resealed with new piston seals and dust boots. If you disassemble a rear caliper and it's seriously corroded, sure, replace it.