One rock in a photo does not give you the size. Only when it is among other rocks can you tell the size, bigger or smaller (scale). By putting one small rock in the foreground, the rocks are shown to be larger. Using smaller rocks in the background gives perspective. Nothing is added to the painting to tell you how big the rocks really are. That is for the viewer to decide. Are the rocks in the background small or large. Only when the painting is finished, if there is something to give the viewer something familiar can they decide the size of the rocks.