Predicting Size of Puppies

Question
Hi there.  Is there any TRUE way to predict a puppy's adult size?  I have read about doubling the weight at 4 months to predict but it doesn't seem accurate.  I have a male German Shepherd pup who weighed about 33 pounds at 14 weeks.  His mother was 70 pounds (pure West German) and I saw the father (tall/big paws) but I don't know his weight (pure West German).  My dogs paws are huge but I've heard that that doesn't matter.  Any help would be greatly appreciated. Thank you.

Answer
After 17 puppies since 1992, I should have a good idea if I had paid attention. I will say that he will be less than double his weight at 4 months.  I think German Shepherds shoot up faster then Labs, perhaps half their adult size at 3 months.  I am quite certain my Lab, Holly, will be a runt, 28 pounds at 4 months.  So I don't think yours is going to be a fine, big boy like the one my daughter had, 88 pounds at a year.  

Switch him to adult chow in a few weeks.  That will delay reaching his full size, giving him more time to develop the sturdy joints he will need over his life.  Likely as a Shepherd he is on the bony side, and I don't need to caution you not to let excess weight stress his young joints.  Remember lean is healthy and don't worry unless the ribs really stick out.