According to the official NBA Rules under Section II - Equipment:
b. A transparent backboard shall be marked with a 2" white rectangle centered behind the ring. This rectangle shall have outside dimensions of 24" horizontally and 18" vertically.
13" x 13"
Answer.
As far as the NBA goes, Rule 1, Section B, Article 2 states: "A transparent backboard shall be marked with a 2" white rectangle centered behind the ring. This rectangle shall have...
It's a rectangle NOT a square, with 2" thick line. This rectangle which is centered behind the ring shall have outside dimensions of 24" horizontally and 18" vertically.