I just bought this backpack carrier for Misha.
Misha is 22lbs/18 inches neck-to-tail and is pushing the upper limits of the bag. I would hesistate with his size if he weren't such a calm dog. That being said, the pack is awesome. It distributes weight incredibly well so I'm not sore after using it (used it a couple times for 30/45 minutes). I even tripped and caught myself without feeling unbalanced at all. Misha just turned 14 and I really like hiking so now he can come along on my longer trips and ride partway. I ended up buying a larger hiking bag with the insert separately so that he would fit. The pack can also be used as a child carrier.
I was worried about buying this because of the cost and possibility of Misha being too big, but I'm really glad I did. Much easier to get through crowds and the woods than a stroller would be