This was the softest bed I have ever felt and my Little One loved it but although I ordered the Medium, which I thought would be big enough for my Westie, sadly it wasn't.  He needed the next size up, so we got him the large one and now he has room to stretch out and snuggle up under the fluffy sides. In the medium one, his back end hung over, as you will see in the attached photo. Its one of those items where you probably think, should I, or shouldn't I .....Well you should because you won't be disappointed once you have the size right. 

I would say the Small is fine for cats and some small puppies, the Medium is ok for smaller dogs like Yorkshire Terriers or small Jack Russells, and the Large for Westies and the more solid smaller classed dogs. My brother has an English Bull Terrier and he only fitted half way on the Large one. These sizings are just to give you an idea, they are no way all tested.