For Youth Players
Youth tennis rackets typically range from 19 to 26 inches in length for players between 2 and 12 years old. As a general rule, the appropriate size racket length for a child is the distance from finger tip to ground when standing upright.
http://www.tenniscompanion.org/kids-tennis-racquet-sizing/
http://tennis.about.com/od/forjuniors/a/raclengthjr_2.htm
For Teen/Adult Players
http://www.wikihow.com/Choose-a-Tennis-Racquet
http://www.tenniscompanion.org/how-to-select-the-perfect-tennis-racquet-grip-size/
