Please measure length,neck,and chest of your pet, It is better to choose the sweater size based on the chest of your pet,The measurement chart is as following:
XS(X-Small): Length 8.7" - Neck 7.9" - Chest 12.2""
S(Small): Length 10.2" - Neck 10.7" - Chest 13.0"
M(Medium): Length 12.2" - Neck 13.0" - Chest 14.6"
L(Large): Length 15.4" - Neck 15.0" - Chest 19.3"