How to identify which brake master cylinder?

As I'm nearly over the drama of changing the exhaust manifold gasket and while I'm waiting for the manifold flanges to be skimmed, I'm tackling the brakes.
the first issue I have is the pedal doesn't return whenever the brakes are pressed so I'm changing the master cylinder but I found about five different types so how do I determine which one I need?
They seem to be catagorised by the number of ports which i can count but also by the cylinder diameter so where is that measurment taken?
Also, if I get one with a larger diameter, will it mount on the servo and would it offer any improvement to overall brake performance?
Thanks

the size is cast into the MC somewhere usually