My kids are both small for their age.  My daughter is now in fifth grade, but when she was in first and her brother in third she was bullied.  The bullies were three fifth grade boys who were teasing my daughter for wearing dresses.  My son, who was smaller than most of the boys in his sister's first grade class, marched up the them and informed them they were going to leave her alone.  They told him he wasn't big enough to do anything about it.  He did not hit them or yell at them.  He just stated "that is my sister and if you do not leave her alone you will answer to me."  Something about the way he stood made them back down.  A teacher's aide who knew my dad as a child said she had a flash back to their school days.  My son entered the sixth grade last year, still smaller than most kids his sister's age, and was being bullied by some older kids for his size.  One of those three boys heard them and informed them that he may be small, but he isn't to be messed with.  He is still picked on once and awhile and he just gets that stance, what ever it is, and they back down, no words or fists needed.  He then tells the nearest adult and goes on his way.