We need a bit more context for what is going on in your head.
Based on your earlier post, I assume the problem you are trying to solve is what sounds like a leak.
Did your mechanic find a leak in the master cylinder?
If you are looking at replacement MCs, it needs to have the right physical connections (mounting flange, hose connections, etc). You can change the bore size, but doing that will affect both the travel and feel of the brakes.
If what you have is a C3 Corvette one (which is extremely common in conversions) and you were happy with it until this new problem developed, I would stick with that.
If you know the sizes of your slaves (if drums) or pistons (if disks) then you can calculate the effect of changing the master cylinder bore.
