Rockys fits really well and is so tight on his belly/ chest (not too tight of course) that he won't wee on it.
You have to measure from the point where the neck just ends (which usually is at the withers) until the point where the tail starts.
Haven't gotten the 9inch one, as Rocky is way too big for it. He is 3.6kg. Maybe someone with a smaller Chi and a Hurtta coat can help you. Also, you could always write them an email I am sure they can help you about sizing .