Don't have any pictures, but I bet that's one cute pup!
I've seen Aussies anywhere from 35 to 50 pounds, and Shelties anywhere from 25 to 40 pounds. Quite a fair size range. If I was to bet, I'd put my money on about a 35 pound dog, maybe 40 at most, that would come about to your knee. And it'll be a smart one to boot!
i dont have any pics of an older one but i do have an 11 week old aussie shltie mix and the lady i got her from said she shouldnt get any bigger then about 30-35 lbs. so about mid calf to the knee. i hope that helps