[Shankenstein]

While many Brembo setups use similar pad shape and caliper type... don't ignore Mike's comment ("All calipers are vehicle application specific").

The piston configuration is unique to each vehicle. Lots of things go into choosing the bore diameter(s), location(s) and features.

Your master cylinder displaces a specific volume of fluid and is designed to have a working pressure range. The vehicle's front/rear weight bias, total weight, and OEM brake bias will also dictate the piston bore(s) total cross-sectional area. The rotor and pad thickness will affect the max excursion of each piston. The force distribution on the pad will determine the modulation behavior (including the assist and anti-lock dynamics). You can also delve into exotic materials and venting for pistons, if you expect ultra-high temperatures. Also, the centerline diameter of the pistons will change with rotor size, since calipers are developed to accommodate a range of rotor ODs and thicknesses.