Size constancy is one type of visual subjective constancy.[1] Within a certain range, people’s perception of one particular objects’ size will not change completely as neither distance is changed, nor the video size changed on the retina. The perception of image is still based upon the actual size of the perceptual characteristics.
According to optical principles, for the same object, the size of image on the retina changes as the distance from the object to the observer changes. The greater is the distance, the smaller is the image objects in on the retina, and vice versa. When a man is observing an object, although the distance of observation is different, but the perceptional size is similar to the actual size. It is caused by past experience, and stimulations such as distance of observation. As size constancy is related to distance, experience and environment closely, if one or more characteristic(s) change, people will be puzzled to observe that object correctly.[2]
An example of size constancy is Müller-Lyer illusion. Human's perception is influenced by the environment (the arrows that makes people view these arrows has different length).