How big the MC's are, depends on the piston area of the calipers.
To calculate piston area, use the following equation: πr2 (3.14159 x radius squared.)
If using calipers with more than one piston, calculate the area for each piston, and then add the areas together. If the caliper has pistons on both sides of the rotor, disregard one side.
Caliper Piston Area (in2) - Master Cylinder Bore Diameter (in)
Up to 3.6 - 5/8
3.6 to 4.2 - 7/10
4.2 to 4.5 - 3/4
4.5 to 4.7 - 13/16
4.7 to 5.5 - 7/8
5.5 to 5.9 - 15/16
5.9 to 6.3 - 1
6.3 to 7.6 - max 1⅛
Often, the piston diameter is known and specified in millimeters.
Example: 45mm rear single piston calipers. So:
45mm diameter = 22.5mm radius
22.5 millimeters radius = 0.885826772 inches radius
Piston Area = 3.14159 * 0.885826772 inches * 0.885826772 inches = 2.465 in2
Thus, a 5/8" bore MC is called for in this example.