I think the 40 in circle is relative to what kind of tree you are protecting, that seemed to work for me at the time, Ive had them up for a couple yrs now, at first that was pretty big for the young fruit trees that I was protecting but they have grown into them and above them, which allows for deer to browse on them actually but not necessarily hurt their growth much, as for wind, I am in a little couple acre clearing greenfield so I don't get wind damage much, that would be something else to consider in your case, but even with some wind, as long as the branches are within the main cage it will sort of protect the trunk from getting beat up and damaged to much, young trees tend to be pretty resilient and take a good bit of punishment.
I haven't had too much damage from voles and don't have gophers in this area although armadillos are just starting to be a little more common in the area, I did have something probably a vole or mole tear up around the base of them pretty bad late last fall, I guess there were a good bit of grubs and such under the mulch but none of them seemed to be effected by them, they all came out and did great this spring.
