Crazy as it sounds, smaller dogs are the most likely to need the extra insulation. Chihuahuas, toy terriers, miniature pinschers are among the smaller dogs that their bodies just can not deal with cold winter temperatures. Short haired dogs and dogs that are very lean like Whippets and Greyhounds, tend to shiver easily and will enjoy their walks more wearing a coat or sweater.
Dog coats are also recommended if you live in an area where the temps drop below zero. This applies even to large breeds that are accustomed to the cold winters but live in the house. Remember their bellies have no fur and are exposed to the elements. Also, dogs recuperating from an illness or injury may be more sensitive to frigid temperatures, as are older dogs and puppies.
What to look for in a dog coat or sweater?
Most dog coats and sweaters are made with wool or fleece. Some have water-resistant fabrics, like human parkas, and would be great if you lived in a snowy area. Whatever fabric you choose, make sure it's easy to care for. Coats and Sweaters will need frequent washings.
To protect your dog adequately, a sweater should fit snugly and completely cover your dog's stomach (except for the little boys, for obvious reasons), the legs need to be free so they can walk, run and relieve themselves. Full length "leggings" for the legs may be harder for your dog to adapt to and may inhibit his ability to move normally. Make sure the coat or sweater is easy to get off and on.
Most sweaters come in small, medium, large and extra-large sizes. Keep in mind that dog clothes manufacturers are just like human clothing manufacturers in that the sizes differ from brand to brand. You will need to measure (with a cloth tape) your dog carefully (especially for internet buying) so the coat or sweater fits. Keep in mind that hair does matter when measuring for a coat or sweater. The more hair, the larger the coat or sweater. Veterinarians don't recommend sweaters for long haired dogs because of the matting and tangles.