After thoroughly reviewing the size chart, we ordered the X-L for one of our biggest cats Stephen, who weighs 21lbs and has a neck girth of 12" and chest girth of 19", for the purpose of training on the invisible pet fence. This harness is far, far too large for him (I could probably get two of our cats in it at once if they ever dared let me) and is not adjustable. That being said, this harness is very well made: very good materials/fabrics - soft and pliable, yet sturdy. Very well sewn, great design and construction and the straps and clips are of good quality. The harness is edged/trimmed with good reflective trim that is very visible at night. I ordered the orange which is far brighter in colour than the photos - which is nice as it's almost a florescent orange - excellent for visibility. Again though, waaaay off from the size chart AND not adjustable.