Here are some considerations to give you a handle on a puppy's growth potential:

Breed and size of both parents: Two 20-pound terriers will not spawn pups that will eventually stand 27" at the shoulder and weigh 99 pounds. Likewise, 2 retriever-sized dogs will not give birth to pups that achieve a maximum weight of only 20 pounds. It just doesn't happen. Difficulties arise in assessment of future size potential when one parent is big and the other small, though in such cases, the bitch's size counts for more than the sire's.

Paw size: As with people, big feet indicate greater height (and therefore weight) potential. You can't have a 100-pound dog doing a pas de deux on ballerina-sized paws. Again, it just doesn't happen. Neither can you have a little Munchkin of a dog parading around on paws the size of large pizzas. Nature doesn't design this way.

Loose skin: The looser a puppy's skin the more room he has to grow into his "new suit" and the larger he is likely to become.

Growth curve: A relatively undocumented but nevertheless valid method of assessing a pup's eventual size is to plot its growth curve. The really rapid phase of growth occurs from birth to 6-months of age. Thereafter growth slows to an eventual halt at 8-14 months of age (say, 12 months on average) depending on parental breed contributions. Either height at the shoulder or weight can be plotted on a chart to be assessed at intervals. For a dog that is properly fed, weight gain should be fairly steady throughout the growth phase. For example, a dog slated to weigh ~ 100 pounds should gain around 2.5 pounds per week after about 4-weeks of age. A dog slated to weigh half this size will gain weight correspondingly slower. For a 4-6 pound adult, the following weight gain applies: 10 weeks, 1 lb. 9 oz. – 2 lb. 6 oz., 12 weeks, 1 lb. 14 oz. – 2 lb. 13 oz., 14 weeks, 2 lb. 2 oz. – 3 lb. 3 oz., 16 weeks, 2 lb. 7 oz. – 3 lb. 11 oz. And a growth curve really is a curve: It will reach a "knee" when about 65% of the dog's eventual weight is reached and plateau after 95% of eventual body weight is achieved.

The "double-it" formula: Some say that whatever the puppy weighs at 14 weeks double it to get the adult weight.

Height increase versus weight gain: There comes a point of a pup's life at which growth plates of the long bones becomes fused and bone length no longer increases. In humans, growth plate closure of long bones occurs between 17-21 years of age though muscle mass continues to increase until full physical maturity at around 25 years of age. An equivalent thing happens in dogs. Though growth plates close between 8 - 11 months of age, body weight continues to increase until full adult maturity is reached maybe months later in larger breeds. A simple estimation for predicting adult height is that a pup will be 75% of its adult height at around 6 months of age.

If all of the above factors are considered, it should be possible to make a reasonable assessment of a pup's estimated size and weight at maturity. And at least there will be no BIG surprises though admittedly even the best estimations are far from 100 % accurate.