When fighting a tall opponent, one of the very few ways to fight them is to get in close. Stick to their body and be prepared if they try to back up to throw a kick. If you take it from a distance perspective, it's harder for a tall person to hit a short person with their hands, because of shorter range of their arms. The taller person will have the advantage of being able to punch downwards, but if you keep your guard up, it should provide enough defense.
Tips for my fellow shorties:
-Stick close to them. Make it so close that it makes them uncomfortable.
-Keep your hands ready! They will most likely back up and try to kick. If they punch, it will come at you at a downwards angle.
-Lean back and keep weight on your back foot. While the trunk of your body will be closer to jam your opponent, your head will be farther back and less susceptable to being struck. It also helps, because with weight on the back foot, you can quickly snap off kicks with your front leg, which do very well in sparring. With a lot of weight on your back leg, you can also retreat faster, if your opponent happens to come at you.
-Be aggressive! Unless you come at them, they will hit you. Most likely hard. Don't give them time to think, or room to attack.
-How to advance: with your weight on your back leg, you have a lot of pushing power. Push off with your back leg and let your front foot come off the ground enough that it doesn't touch. During this, you'll clear the ground slightly and advance quickly, but clear only a short distance. This can be done rapid succession, so you can advance quickly, and retreat just as fast. Since you advance in short bursts, it's more advantageous than lunging, and has the added effect of psyching out your opponent. This may not work for everyone.
This next tip I discovered tonight when sparring, which actually might be of some use, even moreso when fighting a nervous and/or wired opponent. When sparring, tap the opponent's forward hand with your forward hand; it will startle them into attacking, more often than not a kick. It doesn't even have to be a powerful slap, just strong enough that it pushes their hand out of the way. Speed is a key factor, however. It doesn't matter which leg they have forward, but it does help if they are in a mirror stance (Your left leg, their right. Your right leg, their left).
Again, these are only tournament rules-based.