If you've been looking at pince-nez for some time, you've run across the Eyeglasses Wearhouse website. They are highly ranked on Google and they have a sizable collection of pince-nez for sale. Prices are not listed so you have to email them. I found their prices to be on the high side but not outrageous.
One thing of notable interest is that they have sizes listed for their pince-nez in terms of millimeters. Then I started to feel like an idiot. I don't understand how one can measure the size of a hoop spring.
I wrote to Eyeglasses Wearhouse on how they measure a hoop spring. I received the following response. "Spread the spring by placing it over one of your fingers. When the lenses are parrell [sic], measure the nose pad width. This width can be decreased or increased slightly by adjusting the shape of the spring."
This is nonsense. How can one reliably measure the distance between nose pads as these are three dimensional objects covering a very small space? Furthermore, each pince-nez will sit differently on a wearer's nose. I discussed this subject with my friend who has many years experience with pince-nez and he agrees with me. There is no consistent and reliable method of measuring pince-nez for fit.
The same principle holds true for fingerpieces as well. The only part that can honestly be measured is the bridge itself. But since the fit is in the nose guards, the bridge measurement alone cannot assure a good fit.
When you see a dealer list a size for a pince-nez, be skeptical. Regardless of the purported size, the mounting may not fit you. This is the reason I insist on a return policy when buying pince-nez.