My dog is a 7 pound chihuahua mix. Based on the size chart, I purchased a size extra small. When I put it on him I had to adjust the Velcro as tight as it would go and the harness vest was still a little big. I found that, depending on the manufacturer, my dog usually wears anywhere from an extra small to a small, so I was surprised that this was big. But, he can still wear it and not get out of it since there are separate adjustments at the neck and chest.I like this harness vest because I find it quicker to put on than a regular harness - and easier to figure out how to put it on. I am pretty sure I have put some of his other harnesses on wrong based on how uncomfortable he looks. This washes well and the Velcro continues to hold tightly. And of course, he looks pretty handsome in it. He now has 2 harness vests so I can wash one and still have the other to wear. Despite the size issue, I am happy with my purchase.