Pi calculated ruler for circumference to diameter with recommended sizing. use it by wrapping it around and wherever the zero line falls is what the sizes you should get.
links to tape measure
Do not scale when printing leave at 100%
Example: if your measurement looks like this I would get the 35mm retainer and/or the 40mm CAR-1. Size down on the retainers and size up on the CAR-1.
All of my devices are sized by the outer diameter of the inner cone. This is the 35mm car-1.