Riley,
I know that the diameter of the loop is about the size of a fist for rabbit, that's what I had luck with in the past. Then again, your fist might be larger or smaller.
Height off the ground, besides trial and error, I think a nice pair of high-powered binoculars and some quiet time would be in order. Watch them closely and I think you can adjust your snares from that information and see what happens.
Here is a plug, get pencils and Fisher Space Pens. I have not tried out the "Travel Pen" that Jeff Randall has on the RAT Cutlery Website but I might have to. Also Rite-in-the-Rain tablets and Moleskine notebooks are wonderful for drawing pictures and recording various bits and pieces of information. Takes the memory out of the equation in the future...
Although Mark Wahlberg makes me want to hurl, whoever did the work-up for the movie
"Shooter" sure did it good. In the beginning of the movie, you see a Moleskine and they have a beautiful drawing in that thing, of the terrain. It really is great and that is the kind of stuff we should all be doing when we are outdoors doing this stuff, experimenting, etc.