What is the Front Brake Master Cylinder Piston Diameter?

[Subtle Focus]

I am assuming that the rims I am putting on my bike are off a 77-78 model or 80+ model I will post a picture as well. I need to order a master cylinder and there are universal ones that come with everything needed to hook it up the only thing I need to know is if the piston diameter is 13mm or 16mm or does it matter?

-SF