S 2.25" x 2.75"
M 2.5" x 3"
L 2.75" x3.5"
XL 3.25" x 4"
I ordered the XL which ran big. Remember that dog boots will all look small compared to human shoes but you should measure your dog's feet to make sure you order the proper size. :)
I think STP had a chart that had you measure your dog's feet. I bought M for my dog who weighs 60# and has big feet! but M was too big for her rear feet. I bought a set of small clogs and use med on two front feet and small on rear feet.