How do i predict how big my puppy will get?

I have a 5 1/2 month old puppy that I just recently got from the humane society. he is a smooth collie mix (they don't know what else he is mixed with). He was checked about 2 weeks ago and he's 22.04 lbs. I know it's hard to predict with mixed breeds, and the best way is to look at the size of their paws, but i just can't tell. his paws aren't exactly oversized for his body, but he does have pretty long legs. Here is a picture of him, it's the best i could find that shows his paws. http://photo.ringo.com/247/247012718O680835138.jpg thanks!!