If you know the father and mother of the shih-tzu, it's usually an average between the 2 weights. If it's female, probably no more than 8 to 10 pounds, depending on the diet you provide. If it is male - male dogs are typically larger than females - he could end up around 10-14 lbs (again, dep. on diet) . There have been exceptions of course, like all things pets, we never know FOR SURE. Just like people, they are always changing.