Sizing: 

Small - Chest: 11-18" Neck: 9-11" Fit: 7-12 lbs 

Medium - Chest: 14-22" Neck: 11-14" Fit: 12-18 lbs 

Large - Chest: 18-27" Neck: 14-16" Fit: 18-26 lbs 

X-Large - Chest: 22-32" Neck: 16-18" Fit: 26-36 lbs