They do have dimensions on the RBI website. Large is 13.125"x9"x3", Medium is 9.5"x7"x2.375", small is 5.5"x4"x2.125". XL is 16.25" x 11" x 4.5" and is one of the only ones that specifies door size 4" wide x 3.25". My 1250g dude actually still even fits through the door of the medium though and fits just fine in the hide in general. The little weirdo eats any and every time I feed him but still managed to lean out last breeding season because he was on the move, so if he was chubbier vs more muscular he might have issues with the door. He's probably reasonably long for a boy too since I got almost 3' on that snake measuring app maybe a year ago and he's definitely grown since then. I don't think he's done growing so he might end up needing bumped up a size.
tl;dr - I have a big boy who is fat at heart and needs to stop begging me for food between meals but he does still fit in a medium. Srs Pipsy, I don't care how long you stare at me in "feed-me" position between feedings, I'm not going to make you fat. I'm just going to feel guilty.