We got size S for our Chihuahua mix terrier (6.8 lbs) and it fits just right. Sizing up to size M might be a better idea so it won't be too tight. We love it anyways.
XXS Perfect fit for my 5lb Chihuahua
The Size medium fits my 9Lb Chiweenie mix perfectly. She’s gotten so many compliments with it on and it keeps her warm!