Shih Tzus come in 3 sizes the rarest size is called the Imperial size known as T Cup size in most breeds
this ranges from 3-5 lb full grown, we do have them on occassion but it is not the norm here. Imperials are expensive.
Tiny types are generally 7-9 lb range full grown, we have these very often.
Standard sized are generally 10-15 lbs full grown, we also have low end and high end standard sized quite often.
This will better help you to understand the sizes the Shih Tzu comes in, all are very beautiful and there are no health issue with any of the sizes, some are just more compact then others.
The only thing I can think of is when an Imperial is under 2 lbs you do need to watch that they eat and rest properly so they do not get hypoglycemic as with any t cup size, once they are 2 lbs, they should not be wilting away if a meal is skipped or too much playing done..
As with anything that can grow, we can not guarantee size, we can only estimate given there size and growth rate during there 8 week stay here with us.