It is interesting to see how the Shih Tzu compares to others. As you'll see, the Chihuahua is the smallest of all toys, both in weight and height, and the Pug is the largest. Do note that height does not include the neck or head; it measured from floor to withers (top of the shoulder blades).
One interesting element is that the Shih Tzu can be the same height as the Pomeranian. However, in regard to weight there is a big difference, 9-16 pounds compared to 3-7 pounds. This is because of body shape and construction.
Size and Shaping of the Shih Tzu
The Shih Tzu has relatively heavy bone set and is rather compact. This is covered in the breed standard as so,
"... must be compact, solid, carrying good weight and substance."
He is almost as tall as he is long. Per the AKC standard, "Length between withers and root of tail is slightly longer than height at withers."
If you add in his height that includes the neck and head, he will - overall - be taller than long.
His legs are moderate size, more short than long, "... must never be so high stationed as to appear leggy, nor so low stationed as to appear dumpy or squatty."
The two other elements that lend to a Shih Tzu being sturdy and compact are his wide chest "... Broad and deep", and his compressed muzzle.
To summarize, the Shih Tzu is a sturdy dog with heavy bone set and a good weight to him; this makes this breed a great choice for those wanting a toy sized dog, but with enough substance that he is not overly frail. The Shih Tzu is easy to carry around, yet not too fragile that children cannot play with him.