Estimated Growth Chart for ShihTzu

Breeders can only estimate because various factors goes into what a dog's weight will become, such as how much the dog is fed, the quality of food, genetic disposition in parent and grand parentage, past puppy weight.

The older the puppy, the more accurate the guestimate.
8 months old - bone structure is full grown but will still gain muscle
6 months old - 80% full grown. Imperials can still grow 1-2# between 6 mos - 2 years.

The chart below is a good guess of what a puppy will be when full grown at 1 yo (however, dogs continue to put on muscle and fat over its lifetime). This is 'usually' within 1-2# of accurate - plus or minus. Males usually weigh more than females by .5#. The younger the puppy, the less accurate.

Beware that the toy-breed chart found online that many shih tzu breeders are using under estimates weight. Those toy breed charts are for skinny breeds like maltese, yorkies, etc. Shih Tzu are stocky and will weigh more even though they may not be any taller. 

To compare the toy breed chart calculates a 7# as (wkly weight x #):
2wksx7, 4wksx4.7, 6wksx3.5, 8wksx2.87, 10wksx2.5, 12wksx2.1