The medium Air 2 is a little large for our 5 month old American Eskimo, but she'll grow into it. At the moment she can wiggle around more than is desirable, but it gets the job done. The collar clip is a huge help in getting a wiggly puppy into the sack. We carried it with us when apple picking in the NC mountains, wanting to be prepared in case she got tired. Instead, it proved to be invaluable when Tilly started trying to eat fallen apples, most full of yellow jackets. We were able to fill our bags without worrying about a sting. So far we've taken it along on trail walks, but she has more stamina than we do in the woods!