For racing situations only.
There are a number of equations involved in working it all out.
I can send you the spread sheet I developed but it is pretty confusing, my spread sheet isn't well labelled.
From a first principles POV you work out the braking g, which in combo with your CoG height and static weight distribution gives the longitudinal weight transfer.
Now you know the weight distribution you can work out the reactive torque your brakes need to produce based on your tyre diameter.
This will lead you to the circuit pressure required to produce that torque based on piston size, rotor diameter and pad friction.
That's where piston size comes in.
From that you can work out the M/C bore size based on foot pressure you want (Requires the motion ratio of your pedal box). This of course can be modified based on the bias you have set.

Or you can skip all this and use general rules......The more piston area you have the less circuit pressure you need to run.
The larger the disc diameter the less circuit pressure you need to run.
5/8" M/C for front circuit and 3/4" for the rears works for just about anything :)

Bryan