The Size group is the anticipated size when the pet will be grown.

The Size groups of dogs are defined by Petfinder as follows:
   Small 0-25 lbs, Medium 26-50 lbs, Large 51-100 lbs, Extra Large > 100 lbs

The Size groups of cats are defined by Petfinder as follows:
Small 0-6 lbs, Medium 7-12 lbs, Large 13-25 lbs, Extra Large > 25 lbs