Hi LR
I think previous posters may have missed the point. Apologies in advance if I'm barking mad. The problem is not how to scale correctly but how to know the size of the object. Without some external reference to the object, it's absolutely impossible.
Imagine I take a photo of my 12 inch ruler on a plain background and then display it. It is possible for the computer to understand that the object is a 12 inch ruler (although anything other than simple shapes is still tough for a computer, especially if it's never seen one before!). Now imagine I take that ruler, cut 2 inches off the end and then mark it with identical markings except that they are now 0.83r times the size they were originally. Take the same photo, with a plain background, and display it. The computer will (wrongly) display it as 12 inches.
Or perhaps 3 identical Russian dolls? Again, without external reference, which one is which? Impossible to say with a photo of a doll against a plain background.
See the problem? Without some external reference, or some input of extra data, it is an impossible problem to solve. The only solution would be for the computer to take the photo so that it knows exactly what size the object is by providing it's own reference.
Now think about photos of mountains or beaches or anything that's fractal in nature!
BTW, (almost) completely unrelated. I stumbled across an amazing fact the other day when doing some reading on fractals. When measured on the same relative scale, the surface of the earth is smoother than the surface of a table tennis ball!