Puppies grow quickly and that means their size might still be changing. While your pup’s growth will depend on its breed, many larger dogs may not hit their full size and weight till around 12 to 18 months! For Ruffwear gear, we always recommend sizing based on your pup’s current chest girth to make sure they are getting a comfortable and functional fit. Gear that is sized too large can cause chafing or slipping. We recommend adjusting harnesses or collars to be snug but comfortable. You should still be able to slip a couple of fingers under the webbing after adjusting. For growing pups truly on the border of two sizes, go ahead and choose the larger size. For example, a 3-month-old puppy with a 17-inch chest girth would be right on the border of our XXS and XS harness sizes. We would recommend sizing up to the XS since their current girth fits within that size range of 17 to 22 inches AND the larger size will give them room to grow. For a quick guide to measuring you can check out our How-To Video for Measuring Chest Girth: