This is a wonderful bed and worth every penny. My 10 lb. Shih Tzu, Lola, always seeks out the plushest accommodations and this is one of her favourite spots. But I ordered a small, and she could definitely use the medium. As I write, she's asleep in her bed with the top half of her body spilling out onto the floor.
As long as she wants to be curled in a ball, she's great in the small. But sometimes a dog wants to stretch out. I think the sizing is off: according to the guidelines, she'd even be okay in an XS.