To answer your question: Take a look at the sizing chart on this website - http://ep.yimg.com/ca/I/yhst-48558679348936_2264_33555301 (They do seem to run a bit on the small side - I never dreamed I would need an XL for our Jack Russell but after reading reviews - that's what I ordered. It basically fit her, but it was a bit loose around her mid section. She is 20 lbs).
This is from the website - http://www.wetnoseboutique.com/pusohaxlxxl.html (Amazon doesn't seem to carry this particular Puppia model anymore)
"Comfortable, Cool Polyester air-mesh material.
Practical & adjustable chest belt and comfortable neck.
Puppia rubber label. D-ring on back for leash.
Machine washable, gentle cycle, air dry.
Very gentle on the neck. Your dog will thank you!"