Most returns of dog clothing come down to sizing. Yet two measurements largely determine the right size: chest and weight.
Chest first
Wrap the tape measure around the widest part of the chest, just behind the front legs. This measurement decides whether the garment will fasten comfortably. Weight is supporting information; two dogs of the same weight can have very different chests.
Body shape
On broad-chested, short-backed breeds such as the French Bulldog, a standard cut tends to be either tight at the chest or long in the back, so check both measurements against the size chart. On long-bodied breeds such as the Dachshund, look at the back length.
If you're in between
Choose the larger size. A tight garment restricts movement; a roomy one can be adjusted.