E46 330 Rear Caliper Piston Size

I'm looking for an E46 330 rear caliper repair kit but BMW does not have stock. Brake and clutch centre does have stock but they want to know the piston size or a sample. Does anyone know the piston size? REALOEM does not list a size on the caliper piston.

Or else, does anyone have a rear caliper repair kit for the 330?