I haven’t tried it, but I love the idea. I’ve always used https://eleif.net/photomeasure, where you can set a known dimension. For example, here I set 10 mm as my known dimension, and now the measurement is 4.56 mm instead of pixels.
Thank you @jwidness, inspired by this site I added the possibility to change the value of the first “known” measure, and the relative length of the other measures will change accordingly.
The extension is now less intrusive, also: it injects code on the page only if you open the app popup.