PFD sizing:

Measure the widest part of your chest.
S/M = 31"-37”
M/L = 38"-44”
L/XL = 45"-51”

 

Birddog Sizing: 

Size	Weight	Girth	Length	Suggested Breeds
XS	5-15 LBS	13"-19"	9"	Chihuahua, Pekingese
S	15-25 LBS	17"-25"	12"	Pug, Pomeranian
M	25-45 LBS	21"-31"	14.5"	Border Collie
L	45-75 LBS	26"-35"	17"	Labrador Retriever
XL	75+ LBS	30"-39"	18.5"	Great Dane, Mastiff