is my dog growing right

Question
QUESTION: i have a German shepherd who was born on good Friday of this year April 6. now i don't know if he is the right size, he measures 21 inches almost 2 feet and he is going to be 4 months.... is he the right size

ANSWER: Do you mean 21 inches at the shoulder as dogs are commonly measured?  That may be a little on the small side.  Shepherds tend to shoot up early and then level off.  This would be a good time to switch him to an adult chow.  That will slow reaching his final size, but slow growth giving him more time to develop the sturdy joints he will need for a long active life.

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QUESTION: from shoulders will be 15 do you think is good...
if not any suggestions to how i can improve he's growth

Answer
That sounds a little on the small side.  However, he is genetically programed to reach a certain size.  As I suggested in my first answer, you should end up with a sturdier dog if you delay it by switching to the less rich adult chow.  There is nothing you can do to make him turn out larger in the end.