None of the piston sizes are the same. They are always larger in the front and get smaller going toward the back, repectfully (largest pistons are always toward the front). The below concerns the 944/968 with brembo brakes.

86-95 rear, all
30/28mm pistons
98mm pads
299x24mm discs

86 (only) front, axial
36/34mm pistons
98mm pads
298x28mm discs

87-95 front, axial
40/36mm pistons
98mm pads
298x28mm discs

88-95 front, radial
44/36mm pistons
132mm pads
304x32mm discs*

*this can be replaced with the factory 'drilled' 322x32mm 928GTS discs along with the larger 928 GTS calipers.

NOTES: A rear caliper will not fit the front spindle. The 87-95 calipers will fit the 86 spindle (to upgrade the calipers). The 98mm pads, front and rear, will interchange.