I ordered a small for Indy and a medium for Murphy. To be honest they seemed to prefer the small one, and that lasted about 3 days. They shared the medium one and that lasted about 4 days, mind you they are ferocious chewers. When I originally rang the company, the lady I spoke to said that is quite a lot of variation in the size range. I'm nearly sure she said they were sized by weight, so I reckon I must have had a large small, and a small medium, if that makes any sense?