What Is Special About a Small Dog Harness?
A small dog harness is a harness designed for small dogs to keep them safe when they’re out and about with you. Some aren't much different from larger dog harnesses, just smaller, but others are specifically tailored to the requirements of small dogs or to better fit small bodies. And all of those small bodies aren’t the same -- a Pug and a Dachshund have very different fit requirements.
Yes, harnesses are good for small dogs (as well as dogs of all other sizes). They're particularly good for small dogs who pull on the leash as they reduce the amount of pressure on the neck.
This entirely depends on the type of harness. Check the instructions if you're not sure how to put on your dog's new harness.
You'll usually need to measure your dog around the neck and around the chest girth (a little behind the front legs), but double-check before measuring as some harnesses require different measurements to find the right fit.