Sparring... OK.
The toughest competitor I ever faced was the person who was the same size as me but stronger and faster.
Where to look... The old saying, "look 'im in the eyes", never worked for me in sparring or fighting. The eyes can deceive. (And, in a real fight, it can give humanity. You don't want that.)
Generally speaking, the smaller person has to get inside on a taller person to nullify reach. A shorter person inside is in his/her prime reach area while the taller person is tripping over him/herself. You have to instill and fortify your mindset to get inside. You do it over and over again and take your hits along the way. Soon, you get in more and more and get hit less and less on the way in.
To get in, you need to develop explosive speed and movement. Kata alone won't do it. Think/use ploymetrics.
Work on timing off the other's attack. If you can move into the attack of a taller person, you nullify his reach and increase your power at the same time because he is running into your counter. Think/practice reaction drills. Again, you need to pay the price in practice.
Another timing tactic, think practice/getting even smaller in response to a taller person's attack. Believe it or not, taller/larger person has a harder time finding the smaller vulnerable targets of a small person. If you compress, it makes those targets smaller and harder to hit. This means, of course, you have the fortitude and/or conditioning to take it off the arms, shoulders, legs or whatever. Explode off the initial contact and, once again, you're inside where you want to be.
There are other minor tactics too but these depend on the character of your opponent. For example, I don't like chasing tall opponents because a lot of times they can outrun you even when backpedaling. But some taller folks are simply not coordinated or as fast. Noticing this, exploding and chasing is a good tactic. High kickers... with good timing, you can duck and come underneath. I have dropped by share of good kickers on their backs/heads with this one. (With friendlier opponents, I have tapped in the groin just let know they could have gotten worse.) With stronger but slower opponents, use fakes/feints to get them to commit then counter after that and get the heck back out.
There is no getting around it. You can do various drills to prepare but then you simply have to spar -- a lot -- to know your own capabilities, to be able to read the other, and to strategize accordingly. The adequate fighter is the one that fights the same way well no matter his/her opponent. The good fighter actually assesses and adapts quickly.
david