If you know the mother of the dog then that's a sure indicator on the size that the dog will get when fully grown.
But if you've bought a puppy without it's mother in sight then you can determine the size of the dog by the size of it's paws.
If the puppy has big feet then it's a great indicator that the puppy will be a pretty big dog but if the paws are not that big then it may be a smaller dog.
You can also look up the dogs breed online to see how large the puppy will be as a full grown adult dog.