The diameter of each mouthpiece is marked in thousandths of an inch, for example .655, which is a 3C diameter, or .686 which is a 1C size. A Marcinkiewicz Shew 1.5 which is a small diameter is .625. Marking the actual size makes the most sense especially compared to the crazy Bach and Schilke numbering systems which are definitely NOT what the brochures say they are.
I measure the diameter .050 down from the top edge of the rim. I have a calipers rigged to find that distance down into the cup. That is the point where MOST rims blend into the cup.
This works for almost all mouthpieces, including super high alpha angle piece like the Shew 1.5 and also low alpha angle pieces like the GR 66L. When I measure a GR I get the same size as marked. "GR" is a registered trademark of GR Technologies, LLC. You will find places on the internet that may give different readings for say a Bach 3c, and that is because they are measuring at a different distance down from the top.
Rim contour can make a certain size FEEL bigger or smaller, so that needs to be taken into account. I describe that on the “Rims” page.
One “size” is generally considered to be about ten thousandths. So in GR terms, you have the 66 (.660) 65 (.650) 64 (.640) etc. "GR" is a registered trademark of GR Technologies, LLC)
Backbores are marked with a number near the top which is the inner size, and a number near the bottom which is the shank size in “Reeves Sleeve” terminology