From the Official Rules of the Womens National Basketball Association (WNBA) published in 2014, the requirements are fairly simple and straight-forward.
RULE NO. 1—COURT DIMENSIONS—EQUIPMENT
f. (1) The ball shall be an officially approved WNBA ball with a weight not less than 18 ounces nor more than 20 ounces. The circumference of the ball shall be within a maximum of 29 inches and a minimum of 28.5 inches and between 71/2 and 81/2 pounds pressure.
(2) Six (6) balls must be made available to each team for pre-game warmup.
Womens National Basketball Association – http://origin.wnba.com/media/2014-wnba-rule-book.pdf
Photo – www.oddsshark.com/wnba