There are some references to the circumference of an NBA basketball; however, there is nothing about circumference in the rules. NBA only lists the pressure in the rules governing equipment.
Official Rules of the National Basketball Association
Rule No. 1—Court Dimensions–Equipment
f. (1) The ball shall be an officially approved NBA ball between 7 1 /2 and 8 1 /2 pounds pressure.
(2) Six balls must be made available to each team for pre-game warm-up.
Official Rules of the NBA 2013-2014 – http://www.nba.com/media/dleague/1314-nba-rule-book.pdf
Photo – www.matchwinner.co.uk/ball-pressure-gauge.html