The U Haul trailers use different size trailer balls depending on the size of the trailer you rent.
The smaller U-Haul Trailers usually use 1-7/8" hitch balls while the larger trailers use around 2" trailer hitch balls.
You can buy trailer hitch ball mounts that go into a receiver hitch that have both the 1-7/8" and 2" trailer balls so that you will certainly have the exact trailer ball for the trailer you're renting from U-Haul.
You can call U-Haul and ask them directly on what size ball the trailer will require that you're renting from them.
But it can't hurt to have both trailer hitch ball sizes just so you can make sure you have the correct trailer hitch balls.
Trailer hitch balls are pretty cheap to buy.