You can get an idea of how a wig looks on a model by looking at the pictures, and it's relatively easy to understand the terms short, medium and long. But, to get a better idea of a wig's measurements, you need to decipher some hairdressing terms.
The fringe is the length of the hair at the front hanging over the forehead (commonly called 'band' in American English).
The crown is the top part of someone's head on the upper back of the skull. The length of the crown is the length of hair coming out of this area.
The nape is the back of a person's neck; on natural hair, this is where the hair finishes at the neck. On a wig, this is the length of hair from the base of the cap.