Your Escalade uses 51mm pistons. Centric part number is 143.66013
Pistons are all application specific...not necessarily looked up by size.
When a caliper is rebuilt, the piston size does not change. If the bore is bad enough to where it would require an oversize piston and machine work, the caliper is scrap. Remember, the piston is the sealing surface...not the bore. (exception being a few early domestic 4 piston calipers and dunlop calipers)