Bill: To properly size the M/C you must provide the piston diameters in the calipers, the number and of pistons in each caliper. You then factor in the pedal ratio and approximate weight of the car and rotor diameters. Wilwood, Tilton and other brake suppliers can help determine what diameter M/C is needed with the above info, don't guess at this do it once, do it right.
From Wilwood website:
What master cylinder bore size do I need?
A:
The master cylinder bore size that you need is dependent on several variables. Choosing a master cylinder bore size begins with defining how the master cylinder will be actuated, manually or with the assistance of a booster, either hydraulic or vacuum operated. Once you have decided on how the master cylinder will be actuated, the information below can be used as a guideline for selecting the right master cylinder. Keep in mind that auto manufacturers have put many years of experience and a lot of testing into determining the right combination for a given car. When building a custom car with changes to the suspension, brakes, tires, and weight balance; you too may need to do some testing to determine which master cylinder is right for your vehicle.
The combined piston area and piston volume of your calipers and the pedal ratio, are two of the primary considerations. Whether the system is boosted or not is another consideration. Remember that a larger master cylinder bore produces more volume and a smaller master cylinder bore produces more pressure. To determine the appropriate master cylinder bore for your specific application, review Wilwood's Tech Tip Guide. You can also contact a Wilwood Sales Technician at 805-388-1188 or email Sales/Tech Support.
Here is info for Tilton:
http://tiltonracing.com/product-category/brake/