Strictly speaking, standard German Shepherd Dogs do not come in different sizes.
Most healthy ones fall within a weight range of 50 to 90 pounds and 22 to 26 inches in height. However, it is possible to have smaller versions of GSDs.
Here are some of the causes of these size variations including more on where the Miniature German Shepherd breed falls into.
This is the most common source of most miniature German Shepherds dogs seen today. The medium to large standard breed of the German Shepherd is mixed with small dogs like the Border Collie and the Poodle.
They are also sometimes bred with other very small dogs including pugs, corgis, and Yorkies. With this in mind, the best way to describe a miniature German Shepherd is a mix between the GSD and a small breed.
These mini GSDs take more after the German Shepherd in basic features like facial and skull traits, coat type and colors, and the general build. With these they end up looking more like shrunk German Shepherds than actual mixes.
On average, these miniature German Shepherds vary in size weighing about 25 to 50 pounds and measuring 15 to 20 inches.
Dwarf German Shepherds on the other hand are purebred variations of the GSD that bear the dwarfism gene.
This could also be as a result of Pituitary Dwarfism where the short stature is as a result of pituitary gland insufficiency.
Whatever the cause is, Dwarfism in German Shepherds is associated with major health issues and often results in the shortening of lifespan by 3 to 4 years.