The normal range for pupil size is between 2 and 5 millimeters, although the lighting plays a large part in that spectrum. As people age, their pupils tend to become smaller, according to About.com.
The pupil is that black circle in the center of your eye, in the middle of the colored section, which is called the iris. Light goes through the pupil to reach the retina, which is sensitive to light. The iris grows to make the pupil shrink when the light increases. If pupils remain at different sizes, a visit with a physician should be scheduled, as that can indicate one of several diseases, states About.com.