Sizing guide- in a photo gallery.
The AIR PLUS is a design based on years of research and feedback from loyal customers.  With additional storage, padding and ventilation the AIR PLUS is built for intermediate hikes, bike rides or any moderate outing you can dream up.  Like all K9 Sport Sack® models, it was designed under the close direction of veterinarians, dog trainers and outdoor product engineers to be ergonomic, safe, and strong.  40 LB/18 KG weight limit. 
Detachable storage bag (14"x9"). Don't forget your gear!
Fully ventilated sides for maximum refreshing air flow
Padded back panel for increased comfort and cooling
Thicker, wider, pressure relieving shoulder straps for increased comfort
Mesh dual side pockets for increased ventilation
Ventilated cooling side panels
Over 5 different safety/failsafe features
Adjustable dual side pockets
Sternum strap avalanche whistle clip