Master cylinder capacity (amount of fluid it holds) is secondary to piston size. Most MC's are either 15/16", 1" or 1-1/16" diameter pistons. Which one you choose depends on if you have a booster, 2 wheel disks, 4 wheel disks, or 4 wheel drum. There are several resources on the net to help you choose, such as Stainless Steel Brakes. Master Power Brakes, and Wilwood.