Netball is game based off of the game of basketball in the late 1800s. Netball became an international game in the 60s with set rules and regulations. One of the rules included the balls measurements, listed below.
The ball shall be a Netball, and shall measure between 690 mm – 710 mm (27 inches – 28 inches) in circumference and weigh between 400 grams – 450 grams (14 ounces – 16 ounces). The ball may be of leather, rubber or similar material.
International Netball Federation Rules – http://www.netball.org/images/documents/rules/ifnaofficialnetballrules2011_for_website.pdf
Photo – http://www.spar.co.za/Sports/Netball