Depending on how you load your trailer (more on this below), your car's rear will sag a bit from the tongue weight (i.e., how much the trailer pushes down on the rear of your car). Ideally, you want the hitch ball to be high enough so that the trailer's tongue (the extension of the frame) lies horizontal. Measure the height of the trailer's coupler (the gizmo that's at the forward end of the trailer) when the tongue is level, and compare it to the height of the receiver of your hitch when it's on the car (and add a couple of inches to allow for your car's rear end to sag). That's the amount of "rise" that you will need.

Remember that for stability (i.e., to avoid trailer sway), you want to load the trailer so that there's more weight ahead of the axle than behind it -- put more stuff forward. The ideal is to have a tongue weight (see above) of about 10% of the total weight of the trailer: e.g., if your "smallest box" trailer weighs 1,000 lbs after it's loaded, you want a tongue weight of 100 lbs pressing down on your hitch ball. If you have too much weight in the rear of the trailer, so that your tongue weight is too little (or worse, negative, meaning that the unhitched tongue wants to go up in the air), this is very dangerous -- make sure you rearrange the things in the trailer to get that tongue weight fixed.

You can measure your tongue weight with a bathroom scale, by the way. It's not rocket science.