The official ball size should be 8.5" diameter (26.7" circumference.) Some “jumbo tennis balls” are only 8" in diameter–if you get some of these under-sized balls, DO NOT TRY TO FILL THEM TO THE FULL 8.5" DIAMETER! The balls should have the feel of a regular tennis ball: they should flex in easily when you squeeze them and only bounce to around half the height they are dropped from.
If we mistakenly sent you an under-sized ball, we will exchange it for one that’s the correct size. We will have balls that are the correct size at the Sept. 23 OCCRA workshops.