Cooper is 10 pounds and his girth is 16". I bought the petite/small and it's perfect.
Max is 9 pounds and his girth is almost 14". The petite/small starts at 14" but it was much too big. I bought him a petite, which is the smallest size. I'm not so sure this will work for a dog under 5 pounds.
I have read some reviews where dogs step out of them so it's very important that it fits snugly.
This is who I bought it from. I asked questions and the responses were extremely fast, as was the shipping
PREMIER GENTLE LEADER EASY WALK DOG HARNESS - NO PULL Royal Blue Petite/Small - eBay