This requires that the viewer know the size of the scale beforehand.
Call the value of the size of the scale S.
Use a ruler to measure the size (e.g., length, diameter) of the scale on the monitor screen and call that value U.
Use the ruler to measure the dimension of the track (e.g., length, width) on the monitor screen and call that value M.
The size of the dimension, D is given by D = S * M/U.
Example: Suppose you photograph a Bobcat track and with a coin such as a quarter near the track. Also suppose that you display the photo on your computer monitor and you measure the length of the track as displayed on the monitor as being 65mm (this is M). Next, suppose that you measure the diameter of the quarter as being 28mm (this is U). Next, suppose you either measure the diameter of an actual quarter or you do a web search and determine that a quarter is 0.955" in diameter (this is S).
The length of the track, D, is 0.955" * 65mm/28mm = 2.31".