A tall player's forehand and backhand shots are farther apart than a short player's. So he is usually weaker in the middle area, where he has to decide whether to hit a forehand or backhand. So against a tall player, play aggressively toward his elbow, which is roughly the midpoint between his forehand and backhand. When he's off the table, usually aim slightly toward the backhand; when he's close to the table, usually aim slightly toward the forehand.
A short player's forehand and backhand shots are closer together, and so he may have less trouble in the middle. But he has more trouble covering the corners. So play aggressive shots to wide angles, as well as the middle. (Even against a shorter opponent, going to the middle not only causes problems for him, but forces him to go out of position, leaving at least one corner open.) Sometimes go side to side, other times to play two or more times in a row to one wide angle. Why? Your opponent has to move to cover a wide angle; after making the shot, he moves back to ready position – but if you rush him, you catch him while he's still moving back into position, in the wrong direction.