The "standard" height for mounting a scope on an AR is 1.40" above the rail. Thats the same height as an AR iron sights, and works for most people. That doesn't mean something else wont work, but chances are that height will work.
You can get there with rings alone, or with shorter rings and a riser. Leupold defines their ring height from the flat top to the centerline of the ring. Burris defines their height from the flat top to the bottom of the lower ring and you have to add in the radius of the scope to get the total height.
Both Burris and Leupold have individual rings––they call them "ultra high, or super high, or whatever" of 1.40". The Leupold rings are pretty pricey and the Burris quite a bit less, with others even less than Burris. Check out the Larue sight, and specifically the LT-104 which is pretty expensive ($200) but does allow the scope to be mounted further forward for better eye relief. There are other cantalevered mounts that are less expensive, and there's a very good thread that's been running recently that presents all (most) of the current cantalevered mounts.
You can get to 1.40 inches with shorter rings mounted on a riser. Its simple arithmetic to get the total height. You need to know the actual height of the rings you're buying––not just the manufactuers designation of low, medium, high, etc. The Midwayusa catalog does a good job of listing the actual dimensional height of the rings they sell.