Master Cylinder Bore

I need to get a Master Cylinder, for my 1972 Duster. This is a factory H-Code car, with manual drum brakes all around. Every supplier site that I look on asks about the Bore. Is there any way of knowing this without removing the old Cylinder. The car stops good with the old one, I just can't seem to get the lid to stop leaking. I replaced the gasket with a new one but it leaks even worse.