How about surface tension? A puddle of water on a flat surface will be held together by the surface tension of the water and reach a certain thickness. Take a picture of that, and there’s your scale.
Kinda tricky to do from orbit, but I’ll let someone else work out those details.
Also, what sort of resolve are you accounting for? Some idealized perfect camera? If so, pollen? Bacteria? Viruses? I’m sure a lot of those come in very standard sizes. Or are you looking for some macro feature of the planet?
i’m not following.
If you take a flat surface, like a tabletop, and pour some water on it, the surface tension of the water will tend to hold it together. If you put your eye, or a camera, down at the level of the tabletop, you can see that the spilled water has a certain thickness. I think that value is constant, defined by the properties of the water itself. If you add more water, it doesn’t get any taller, it just spreads out more.
Wikipedia has an equation that defines the height as a function of the surface tension and density of the liquid, and the acceleration due to gravity. Those are all known values. I think that’s what the OP was looking for, something that can be photographed and exists only at one size, defined by physical laws.
well, that is what I am looking for in principle. It is, as you pointed out, kind of tricky to do with an aerial photograph though. 
Hmm. convection cells might work. I have seen some mighty strange ones though!
thanks