Theres no definate answer- some people have strong jaws, some have glass jaws- some people punch hard and heavy, some dont. Simple put, as a small 15 year old girl, it is possible for you to knock a full grown man out, but there are many variables. The best thing I can say is that the most dangerous punch is the one you dont see. Most people who are fighters, be it boxers, kickboxers, or mixed martial artists, have decent jaws and can certainly take a shot when they see it coming. Why they get knocked out is because of the setups utilized by the other fighters- they sneak that uppercut, that cross, or that hook in their perfectly, and the opposition never even sees it. So, in short, if you work your combinations and develop some power, you dont need 1000lbs of force to ko someone