The general rule is to fit your dog’s body entirely. A coat should cover the base of the neck and the base of the tail. Anything that protects your dog less than that won’t be as valuable. But also consider your dog’s fur density. Dogs that grow a natural winter fur when temperatures lower down won’t necessarily need a full-body coat. They may thrive with coats covering their chest and part of their belly, keeping them sufficiently warm. Don’t forget it should also be a coat that doesn’t limit mobility. If you don’t want your dog to feel uncomfortable and struggle when moving around, then get a sufficiently loose jacket as well.