Are there standards/references for determining spatial scale in photographic images?
One possible answer to my own question is provided by the photographic scale reference detailed on this website.
https://smallpond.ca/jim/scale/

It doesn't quite provide the solution I was looking for, buy may help spur ideas. I really need something that can accommodate a good scale estimate even if the reference is rotated in space (about any of the three axes), and is of low pixel resolution.