I got the Medium, my dog is 23 pounds and barely fits curled into a ball. That would be okay if the construction weren't so flimsy; the sides don't stay up, there is barely any padding so it's basically flat as a pancake. I did try to fluff it in the dryer as some reviews suggest but this did nothing-not even for the wrinkles which are significant. For the price this is extremely cheaply made and I would hardly call it a car seat.