A newborn Shih Tzu puppy starts out weighing about 6 ounces or less. These dogs stop growing at about 7 to 10 months of age. At this time, their mature weight usually reaches a total of somewhere between 9 and 16 pounds.
At about one month old, a Shih Tzu should be about 10 percent of its final adult weight. These dogs reach half their final weight when they are about four months old. The ability of a Shih Tzu to grow rapidly depends on a few factors: how active it is, how much it eats, and how healthy it is overall.