We make three sizes of BarkerBag which accommodate a wide variety of dogs. We designed the collars to adjust down to much smaller sizes; the sizes listed are the maximums. Also realize that if you are going to use your BarkerBag in conjunction with your sleeping bag, you will be sharing space, so your dog doesn't need to fit completely within the silhouette of the bag. (I like to sleep with one leg in the BarkerBag for maximum foot warming!) The BarkerBag can also zip closed to be used as a standalone dog sleeping bag, and in this case your dog does need to fit within the footprint of the bag. Please contact us if you have any questions about fitting!