My wife and I are big into backpacking and hiking. I have never hiked/backpacked with a dog a i have never been the primary owner. I would love to take my V. Not sure if anyone out there could help with some size information. I am looking to to get a pack from REI.com was looking at getting the Ruff Wear Singletrak Hydration Dog Pack Medium, but not sure on sizing typically found on dog packs. He is roughly 7 months (51 lbs.) and still has some growing to go. Any advice would be appreciated.