My ninjutsu instructor is 5'2 and a 13th Dan, hes a bloody amazing fighter. Ive never seen him beaten in a scrap.
Im 5'1 and there are two other girls in the martial art shorter than me, and none of us find it a disadvantage, in fact i find it easier to fight the bigger guys as it is easy to use their weight and strength against them. As for untrained females, its been several years since i was one, but i can tell you that a willingness to fight and hurt someone will usually do much more than hight/weight etc. Not many people are willing to seriously hurt others.
But like others have said, it is best not to fight whenever possible.