Do not let opponent get a hold under your arms or on your legs, since he or she may easily dump you on your back. If you are shorter, do not go to the guard position. Opponent's reach and wide base will make it difficult to sweep and opponent's punches reach further than you can remedy by holding off. If you end up on your back, use the guard to get back to your feet, or to climb on opponent's back.
One of the best cinches against such tall person is a seatbelt and bicep tie-up. Move off to opponent's left side, reach around the back and grab belt/pants at right back hip. Put thumb-index arch of your left palm on right bicep/inner elbow. From here, you have control, you are halfway to opponent's back, opponent cannot hit you, you are out of the firing line of his knees, and you are halfway around to opponent's back for a back mount. When you are off to the side, you only have to fight half the opponent.
Dealing with Law Enforcement
Notify authorities of an attack as soon as possible. Cooperate with law enforcement officers and be polite. However, do not make any statements until you talk with an attorney. Any statements you make may be used against you, even if you were not at fault. Be careful of apologizing are saying you are sorry. This may seem the polite thing to do but it implies guilt. Some states have make laws that say apologies may not be used as admissions of guilt.