It will partly depend on the trailer you are towing. Is it an RV, or something else? You want the trailer to be level when towing, so it's best to level the trailer and measure the distance from the ground to the top of the hitch. Then measure from the ground to the receiver. The difference is the approximate drop. If you are going to use a WD hitch, this gets a little more complicated.....
I think it really depends on how high the trailer rides that you intend to tow. The standard 2" drop from my old Xterra ball mount is not enough to level out my small 16' trailer when I use it on the new Titan. I need to go to 3-1/4" at least.
Does anyone know what size drop I need when buying the trailer hitch to slide into the reciever. I have a LE 4x4, big tow, no off road.
I needed a 4" drop, but you should measure. Use the trailer tongue stand or other jack to put the tongue in the right position, then measure from lower level to ground. Subtract this number from the height of your receiver to the ground and that will be the amount of drop you need.
Go to Wally World and buy at least 3 different drops and be sure to keep your receipt. Try them all out with your trailer attached. Look at your trailer from the side to see if your trailer tongue is parallel to the ground. Keep whichever one works the best and return the others the next day.