For Oscar Newman clothing, the back length is most important, but the girth, neck and weight will help achieve a perfect fit.

Size	Back	Chest/Girth	Neck	Weight
XXS	6-8"	8-11"	6-8"	Up to 3.5 lb
XS	8-10'	11-14"	8-10"	4-7.5 lb
S	10-12"	14-17"	10-12"	8-11.5 lb
M	12-14"	17-20"	12-14"	12-16.5 lb
L	14-16"	20-23"	14-16"	17-21"