I actually bought the small and had to return it. Thor is 5.25 pounds but he would not lay in the small for one minute. I got the lg and he jumps in by himself and sleeps in it. I travel every week and he goes with me so we always have a thin little blanket and his favorite toy. I like it because he can move around and streach out like he likes to do. Good luck I agonized over the size also but the bag itself is great.
