OK - well you'll be glad to know that 21 inches is not in the least small, it is absolutely correct for a female boxer. They are supposed to be between 21-23 inches tall full-grown, and your girl will probably grow another inch before she's done (so 22 inches, which is right in the middle of the standard). So if that looks small compared to others, you can be happy in the knowledge that it is those others that are incorrect for the breed, not Daisy 
As for her being skinny - honestly, do not worry. If she eats well and is otherwise healthy and active then she is fine. She will start to fill out after she is finished growing upward. Boxer puppies are notorious for being skinny at this age, but will gradually put on weight in their second and third years.

As for her being skinny - honestly, do not worry. If she eats well and is otherwise healthy and active then she is fine. She will start to fill out after she is finished growing upward. Boxer puppies are notorious for being skinny at this age, but will gradually put on weight in their second and third years.