It depends on if the shih tzu is a show dog. If it is entered into dog shows it will have very long hair that sweeps the floor. There is usually a bow on top of the head, even males. As a pet most people keep the hair short.
The typical style of Shih Tzu cut is shaving the entire body and doing a "puppy cut" on the head, leaving the tail long, keeping it looking like a puppy.
Shih tzus are often either shaved to a very short length or completely grown out. The looks are very different, and the owner may decide between convenience and appearance.
My aunt has an shih tzu dog and she usually keep her dog with a low cut, and the top of the dog's head has enough hair for an ponytail. Petco usually does the hair cut so I believe that's the typical or most popular hair cut for shih tzus.