In my litters, the obviously biggest pup has always turned out as the biggest dog in the litter. For the females, there was more variation. I kept one of the smaller females, who is now the biggest in the litter, I think (not by a lot like the male is, though). So my experience is that the smaller pups generally end up normal sized or larger, while the larger pups stay large into adulthood. In comparison to their littermates. For size, I like to look at the grandparents. If they were big- or small- or there was a lot of variation in size, you'll see that in the litter, more than likely.