6 months old - At this age, you can expect you, Shih Tzu, to be either half its body weight or more, in this case around 4.6kg. 7 months old - 5.5kg is the average weight for a Shih Tzu puppy of 7 months. 1 year old - Your Shih Tzu at this stage should be a fully grown adult and weigh around 5.6kg - 7kg.