The measurement is usually for the height. There are several species of batfish, and they range from fairly easy if you make the effort, to pretty well impossible to keep. The common bat fish seen orbitculus, are brown and look like a leaf while young, but develop black striping as adults and look similar to the large SW angels. Very active fish, and they need a large tank. My LFS has a beauty that's about 18" right now, very social, fast swimmer, in a 200 gallon tank with a lion, a marine betta, and a yellow tang.
There are several other species that sometimes show up in the hobby, but they are not suitable for community tanks, and are usually tough to keep feeding. I would not have a bat in a newly setup tank, and I don't consider them a good starter fish.