What Master Cylinder for 4 wheel disk brakes?

You'll need a different MC depending on power or manual so make that decision first. If you go manual brakes you'll want something in the 15/16 range. If you stay with a booster then you'll want something bigger, probably over an inch in bore size.

Brakes are a hydraulic system so you need to know the size of the caliper pistons before you pick the size of the MC pistons. The piston sizes need to be matched to work together. Big pistons in the calipers require big pistons in the MC to move the fluid. Small pistons in the calipers need small pistons in the MC. You can't pick one without knowing the other. Well okay you can, but you'll be picking blind so you'll probably make a mistake. Kind of like picking a cam without knowing anything else about the engine.