I went through 4 harnesses before I found one to work for Lexi. I love it and I can let her wear it all day so when she goes out to potty I don't have to fight her to put it on! I bought mine from Petco, but they only sell the small not the extra small, so it depends on how small your pup is. The first link is to a site that sells the small and xsmall and tells your the dimensions for both.
Tiny Step-In Dog Harness by Li'l Pals Coastal Pet Li'l Pals Adjustable Mesh Harness in Blue Lagoon at PETCO