How Tall Is The Smallest Shih Tzu Dog?

I have a shih tzu cross miniature pinscher but Shih tzus should stand no less than 8 inches and no more than 11 inches.

First of all Shih Tzu is already plural, just like the word deer, so there is no need to put an s on the end.  As stated above the Akc Shih Tzu standard states that the Shih Tzu should stand 8 to 11 inches tall and be slightly longer than long. A Shih Tzu reaches their maximum height at 6 months and while most females reach their adult weight at 7 to 9 months, males may take 9 to 12 months to reach their adult weight.  All Shih Tzu regardless of size should be height and weight porportionate and be porportionate heavy for their size.  Most of these tiny Shih Tzu still stand 8 or 9 inches tall.  And yes it is possible for weight to be affected by feeding- but except for extreme situations where a dog is emancipated or obese (the owner should be working with a vet in those situations) but you are not going to make a dog designed to weigh 5 pounds weigh 10 and the reverse situation is also true.
To answer the other portion of your question the smallest purebred Shih Tzu I know of stand 5.5 inches tall from floor to withers (shoulder) and is 6.5 inches long. He weighs 2.1 pounds but as he just about a week short of 6 months he will gain more weight.

A shih Tzu dog stands from 8 to 11 inches.