The exact amount of land needed depends on the shape of the land property boundries as well as the distance you want to shoot your rifles. 40 acres would be enough to shoot 300 yards, maybe further, again depending on if the property was laid out more rectangle than square. You need something to safely trap the bullets and also should be concerned with a possible wild bullet going off to the left or right, as well as ricochets from impacts anywhere from the shooting position to the target, and the down range berm itself.
Most outdoor ranges use a pile of dirt to construct down range and side range berms. Many folks go out into the country side on public land and place their targets up against a hill. The hill is used to safely stop the bullets. Depending on the hill and angle this may be safe, it may not. Some land owners just happen to have a hill or two on their property and they use it as a down range berm (back stop) and save on getting a bulldozer to excavate the berms.
More important than the actual size of the property to fit the range, is the use of adjacent properties. How close are the farm or residences. Will the noise bother them.....will the thought of ricochets bother them?
There are many areas in Arizona which have open lands for sale, you could possibly shoot on. And, every major city/metro area is surrounded by these country acres. Question may be......do you want to live out in the country or close to city convenience? I'm not asking that question for you to answer in this thread, just something to think about.