Ball size? You are going to have to check the tongue/coupler on the trailer. Most are 2", some small trailers are 1 7/8" and bigger ones are 2 15/16"
Don't guess, make sure. Too big of a ball and it won't couple. Too small of a ball and the trailer could fall off, I've seen this first hand.
How much drop? Level the trailer with the trailer jack and measure the bottom of the coupler to the ground. Add a couple of inches. The distance between that and the bottom of the 2" receiver hole will give you the drop. Why a couple of inches up? The suspension will squat some and it's better to have the tongue up a little. You could get an adjustable draw bar. Some use pins to change height and some use a bolt adjuster.
My trailers have a hitch that is ~14"-16" off the ground.
So if you measure the bottom of the receiver hole to the ground then subtract 14" that should give you the drop needed. Most trailers that size are 2". The cheap harbor freight trailers are 1 7/8".
You could but a few different size drops and a 1 7/8" and a 2" ball then when you find a trailer choose the right one and return the unused ones. Don't forget a wrench and a hitch pin.
Walmart will probably have everything you need at a good price.
unless ur towing a trailor with some serious tongue weight, leave it alone. it aint worth it. the bigger the ball the stronger. thats all u need to know on that. 2" is pretty standard. i never use a 1 7/8 anymore (for anything).