I use them because they are easy to clean. I think they are made of ABS plastic. My 850g BP can sqeeze into a medium but he will be upgraded in a couple of weeks. For a juvie, I would say go with a medium. The smalls are tiny, about hatchling size. If the medium seems too big, you can fill in some of the space with some balled up paper, or, if you are using wood type substrate, you can scrunch it down in the substrate to make the ceiling lower...