I guess it would depend on how big your puppy will get, not just in weight but in size. As my ZoE is long and tall, so I would go with the medium for her. Unless your puppy is estimated to be very small, I would probably go with the medium too. I got a small in a purse/handbag carrier when ZoE was just a few months old and later regreted it as she grew too big for it and I ended up then having to buy another in the larger size later.
__________________ (ZoE  )(Chelsea ) |