To totally make sure you get the right size, see the chart here with girth measurements. Girth is the measurment that goes over your dog's shoulders, right behind the front legs, and back up again. Please do this with your dog standing. Measure it as if it was the harness - not too tight, not too loose. If the measurement is at the very top of one size, then go up to the next size.

POUND Range	Girth Inches	KILO Range	Girth cm
6-11	11-15	3-5	28-38
9-18	13-20	4-8	33-51
12-23	15-21	5-10	38-53
18-35	16-24	8-16	41-61
25-65	21-32	11-29	53-81
55-120	26-39	25-54	66-99
110-250	34-52	50-113	87-132