Thumb Studs and Washers: Is there one or two standard sizes? How do you measure?

[MTHall720]

I want to replace the washers in some folders, and also replace a few thumb studs with something a little more "custom" looking.
Big questions for me are:1] How do I measure these, and 2] Is there a standard one or two sizes most often encountered with production folders?

Thanks in advance for any ideas on this.