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!