Mars Rocks.. I know, but this one is interesting

If the images were taken by Curiosity Rover (NavCam cameras only - the Curiosity high resolution mastcam is not supported), Phoenix Lander, or MER (Mars Exploration Rover) rovers, you can use AlgorimancerPG (a free program) to find the distance to any given object, and from that calculate the size of it. For example, the Mars "Bigfoot" is a tiny little statue less than 6cm tall, less than 5 meters from the cameras. I did my own stereo ("3D") image of it (the "Bigfoot" statue) to make sure it was not just a trick of light and shadow, but that's a story for another thread.

You will need AlgorimancerPG, and both of the frames from the left and right cameras to do the measurements. It works using parallax (same method they use to measure distances to stars), and measures the pixel difference between the positions of the object in both views to triangulate and find the distance to it. It uses the distance center to center between the lenses of the cameras as the baseline for the parallax triangle, with the object at the apex of it. You can then figure out the size each pixel subtends at that distance, and, measuring the entire object in pixels, convert to height and width measurements at that distance.

More frames are better - it's best to get all the frames you can find from each camera of that object, and do the measurements multiple times, averaging them out for a more accurate reading.