It isn't hard to figure out how big your Maltese puppy will grow. The breed standard for this small white dog limits weight to just 7 pounds at maturity and states that 4 to 6 pounds is preferred. If you don't show your dog, an extra pound or so is acceptable -- but more than that probably indicates obesity and its attendant health risks. The standard doesn't call for a height limit, but most Maltese dogs mature between 8 and 10 inches high at the shoulder.
Perhaps the best way to determine the eventual size of your Maltese puppy is to meet the parents. If you buy from a breeder, note the mother's size when you inspect her. If the father isn't on the premises, the breeder should be able to provide you accurate information about his size. A Maltese puppy born to exceptionally small or large parents will likely mature at either end of the size scale for the breed.
Although genetics greatly influence a dog's adult size, they way you care for the Maltese puppy also plays a role. Feed your Maltese a high-quality food recommended by your veterinarian, and make sure she gets plenty of exercise -- the Maltese is more active than many other small dogs. Consult your vet if your pet exceeds 7 pounds -- if she doesn't need a diet or more exercise, she might have an illness that needs addressing.