As a thieves hideout it looks about the right size. The cave has perspective issues though.
Basically, the cave is four tiles high at the leftmost section while it's only two tiles high elsewhere. You could say it's natural for a cave to have different heights at different places, but the transition doesn't work. Think about how high the roof has to be at different places and things will look weird.
Other than that, it looks great. It's has enough details to not look boring, but not so many it feels cluttered.