There are four types of official tennis balls; the diameter range of three is 2.575" - 2.700", the dimension range for the other one is 2.750" to 2.875" (type 3, the slow ball).
The measuring rule follows:
"In all tests for diameter, a ring gauge shall be used consisting of a metal plate,
preferably non-corrosive, of a uniform thickness of one-eighth of an inch
(0.318 cm). In the case of Ball Type 1 (fast speed) and Ball Type 2 (medium
speed) balls there shall be two circular openings in the plate measuring 2.575
inches (6.541 cm) and 2.700 inches (6.858 cm) in diameter respectively. In the
case of Ball Type 3 (slow speed) balls there shall be two circular openings in
the plate measuring 2.750 inches (6.985 cm) and 2.875 inches (7.303 cm) in
diameter respectively. The inner surface of the gauge shall have a convex
profile with a radius of one-sixteenth of an inch (0.159 cm). The ball shall not
drop through the smaller opening by its own weight in any orientation and shall
drop through the larger opening by its own weight in all orientations."
So it would depend on which type ball you use and the bore diameter of your mortar.