I don't know about knives, but in fencing sword, the first target is the hand and arm, not the belly. The hand is as close to you though for any size of your opponent.

A tall opponent has a longer reach, so that is what you need to be wary of in fencing. It means it does have a advantage to attack. But once the blades crossed, the advantage is gone, and the you have the advantage bellow your taller opponent. Because you have more room for movement in the two blades space than your opponent. You are committed there though, because getting away from the situation will be harder.