Seriously, who the fuck cares if some punk ass kid tells you that you're small? Oh yeah, an insecure douche, that's who.
It has nothing to do with the content of what he is saying, he could call you fat, limp dicked whatever, he could come and shove you, stand in you path and attempt to block you, it really doesn't matter, the intent is the same, he is trying to disrespect and humiliate you. I would have ZERO respect for any man who allowed others to do this to him, for a man to retain his self respect and dignity he must stand against those who try to insult him in such a way. The reasons are two fold, one, it prevents douchebags like this and others from attempting to disrespect or humiliate you in the future and secondly the douchebag learns a vital lesson that his father neglected to teach him.
The question you need to be asking yourself is, Who goes around attempting to humiliate and disrespect strangers? Oh yeah, an insecure douche, that's who. The fact is a young man who is behaving like this has no self respect and others will not take him seriously, he embarrasses himself by behaving in such a way, it is the desperate attempt by an insecure young man to garner some attention, regardless of the detrimental effect it has on his future. The path he now faces to be respected by others will be a long one, it will take a long time to prove to others that he is no longer a fool, since he behaved publicly in such a manner for so long.