There is no way to know until you know what size the trailer needs. I would say 2" if you just plan to rent a uhaul. They adjust to different sizes typically and 2" is stronger than 1 7/8". 1 7/8" is rated for 2,000 lbs almost always. 2" balls are typically rated for 3500 lbs all the way up to and beyond 10,000 lbs. 2 5/16" are for larger trailers and range from about 5-6,000 lbs to beyone 25,000 lbs.
If you don't even know what you plan to tow then either buy both a 1 7/8" and a 2" ball and keep them in the truck or just wait until you know. What good would it do to buy and install 2" ball (even at a good price) if you'll never need it? You can't haul a trailer needing a 1 7/8" ball with a 2" ball. Buying stuff you don't need is more wasteful than buying something at the last minute at a slightly higher price. Plus you can buy a hitch ball just about anywhere including Walmart and Meijer, you're not going to overpay too much if you wait.
