You should be able to find choices that are 22" or 24" by 28 or 30" which would work for the first room. 24" X 36" is another common size (for the second room), but since you only have 37" I'm not sure that would work without being able to see the light fixture. Can you post pictures of each room? That's not to say you wouldn't be able to find something out of these size ranges. Use those as rough guidelines when searching and see what's out there that works stylistically. Once you find something you like, you can use painters tape to put up an outline in the size you're considering buying to help yourself visualize it. Most of these things are returnable too if it ends up not working out.

Frameless or a framed mirror? Frameless can be made to any size by a local glass shop. Pictures would help us help you.