Sweaters are not just a silly accessory. Many dogs need to wear them in cold weather, even inside.
Short-haired breeds, older dogs, sick dogs and puppies are likely candidates for a cover-up. If you'd like an unbiased second opinion, ask your vet what they think.
To be effective against cold, a sweater should completely cover your dog's stomach, keep its legs free for easy movement, fit snugly and end at the base of the tail.
Sweaters come in small, medium, large and extra-large sizes. If you are unsure which size your dog should wear, consider their weight along with their size.
If all else fails, hold the sweater up to your dog to check for size. If your dog is wide and short like a bulldog or basset hound, buy the size that fits around them and then cut the sleeves to fit.
Choose a sweater that will be easy to care for. You might want to avoid dry clean only. If you're the kind of pet parent that likes to go all the way for their little furry baby, consider boots for your dog to protect the pads of their feet from cold.
To help them adjust to their new outfit, have your dog wear the sweater anytime it goes out in cold, wet weather. Make putting on the sweater a part of the dog's routine, like putting on the leash. Then when they see the sweater come out, it's joy not dread they feel.
There are literally hundreds of places to shop online to find what you are looking for, but going to a local pet supply store to shop may be a lot more fun.