The pack-o-pet in small. I LOVE it and so does Phoebe. She's estimated to be about 6-7lbs. Right now she's probably a little over 3lbs and she has plenty of room. Whenever I take her places it in, no one knows she's in there. She also plays in it when it's sitting on the floor at home
Good luck. It took me weeks before deciding on which one and a size.