Hitch balls are available in three different sizes, 1-7/8", 2" and 2-5/16".
This is a measurement of the ball's diameter and should be the same as the coupler of the trailer you plan to tow.
The ball sits on the ball mount and provides the attachment point for the trailer.
If you plan to tow several trailers with different size couplers, you will need either a different hitch ball for each of them or an interchangeable ball system.