That is a very good question. Gillette claims that a typical beard hair is 0.1 mm or 100 microns. I found an interesting article that claims beard hair varies from 17 microns to 180 microns. This article also explains why beard hair is more difficult to cut.

I have calibrated my microscope so that I can take measurements. This means that the scale you see at the bottom left of each photo shows the distance of 50 microns in that photo. Using that scale you can get an idea of how big a beard hair between 17 and 180 microns would be.

(Sorry for the late reply. I haven’t found a good phone app that provides Lemmy notifications.)