Relative Size refers to clues in the picture that help you figure out the size of objects.
Below is a drawing of a fireplace mantel. It has a candle and picture frames. You would imagine that the objects are only about 8"-12" high. Now when you place a human on the mantel that confuses the scene. Either the mantel is much bigger than you thought or the human is a small as a mouse.
How did you do that?
Playing with relative size resources:
Another way to demonstrate that students understand relative size is using animation to show size change from foreground (bigger) to background (smaller) along with the other aspects of creating the illusion of depth. See this lesson for more info.