Forget for the moment that studies show that 80% of rapists will give up if they meet with resistance.
What can anyone do against a physically overpowering foe?
Goliath was huge, armed, armored, skilled, experienced, and looking for a fight. David was young, small, inexperienced in combat, wore no armor, and was "armed" with a staff and a sling. Nevertheless, David had many attributes that he brought to to the match, and they proved to be more important than Goliath's advantages.
Of course, first and foremost David had his faith. Whatever your personal beliefs, it is important to understand that faith gave David a clear mind. He saw the situation for what it was and did not panic. Emotional control, as usual, is the first thing to bring with you.
Because he had a clear mind, David did not fall into the trap of playing Goliath's game. He went into battle with the tools he knew and arranged the fight so it was on his terms. He could reach farther with his sling than Goliath could with his spear or sword. Unlike most people in similar situations, David forced his opponent into battle on his terms.
In a modern confrontation, using the environment is just as important. Can you keep out of range or put obstacles between you and the aggressor?
If he extends an arm to touch your body, can you attack that arm?
Until part 2, I urge you to go over the entries on Killer Instinct, and think about how you can use these principles in a confrontation with a larger, apparently stronger opponent.