well, I've been told a 1" bore MC is the default for manual front disc brakes, so you could go with that.

smaller bore will mean less pedal effort, but you also need to balance stroke volume to make sure you're moving enough fluid.

I did a bunch of math, calculating mechanical advantage from the pedal (lever), and the hydraulic advantange, and figured out that with a 7/8" bore, I would only need 50 lbs of "leg pressure" to get the maximum braking effort. armed with that number in mind, I called up maste power and being the brake experts that they are, convinced me I needed a 1" bore for the stroke volume needed for my disc brakes

so I bought the larger master.

then, later discovered that the POWER brake master was only 15/16" bore! I could have probably gone with the 7/8" and been OK!