It's the standard for indoor bearded dragons since it is large enough to give walking space while still small enough to enable good UVB coverage and heat retention. I know people keep Blueys in them as well.
Pretty much anything smaller than those lizards work. From my experience lizards use all the space so putting small lizards in a big tank isn't a detriment. I've given my young 15cm Water Dragon a 3.5' x 2' x 2' and he uses all of it.