I'm going to be undertaking a karate competition and one of my fellow club members is also attending. They are much larger, stronger and powerful. Is there any way I have the slightest chance in defeating him? Either by technique or by A certain strategy throughout the fight?
Asking for a strategy upfront is not going to be beneficial, especially on the internet - we have never met this opponent so all we know is he's bigger and stronger than you (and we have no idea how big you are).
Train hard. Be confident in your knowledge and capabilities. You should start learning to read your opponents - it's an important skill to have, you won't be able to analyze them all ahead of time. Even if you do know them there is no guarantee they will behave the way you expect on competition day.
Learn to spot and react to telegraphing. Learn to move, you should have practiced it enough that it is instinctive. Visualize what you're doing as you train, so your mind and body both know what the move is for. Most of all, ensure your mind is calm when you enter the match - it might sound cliche but it's best if you're not thinking rather than thinking too much.
My advice whenever you're facing either someone who's more advanced or bigger is to concentrate on your basic, "high percentage" techniques and be as precise and as quick as you can. Stick to things you know well, and avoid any temptation to do something "cool" or something you've just recently learned.
In another answer, I discussed what "high percentage" techniques are:
By "high percentage", I mean things that have a high ratio of scores over attempts in actual competition sparring. As opposed to "low percentage" techniques and strategies which have a low ratio of scores over attempts.
Specifically, I said this of point sparring:
As a general rule, striking techniques which can be used in the most situations and are fast, direct, and simple to perform will be the highest percentage ones. For hand techniques: jab punch for boxing or reverse punch for karate and Taekwondo. For foot techniques: low/mid-section round kick and front kick. I would also add side kick if you're talking about Taekwondo in particular, but not for most other kicking arts (Taekwondo really likes to use that kick).
So your main focus for preparing for this tournament should be on basic techniques - the ones you learned in white belt. That would be the basic jab punch, the reverse punch, the round kick, the front kick, and the side kick. Keep your kicks to the mid-section, rather than aiming high. Work on your blocks and keeping your guard up. And stay active, always moving sideways (not backwards), to the less dominant side of your opponent (if he's right-handed, that means moving to his left side, which is your right side). Staying active keeps him from getting a lock on you as a target. And moving to his less dominant side means he'll be less precise, slower, and not as strong.
This sounds boring, but boring wins.
Another thing to do to help prepare you is to try sparring bigger opponents. You may find that the distance you need to maintain (due to his longer reach) makes it hard for you to get in and land a kick or a punch. You're going to have to find the right timing that will allow you to enter, kick or punch, and get out without being hit.
Usually the best timing in general is "half-beat" timing, as opposed to "full-beat" timing. Full-beat timing is where your opponent does something, you block, then you do something, he blocks, and it repeats. In "half-beat" timing, you begin your technique when your opponent has just started doing his. It causes him to have to stop what he's doing, figure out what you're doing, recalculate an appropriate response, and then execute it. As you might realize, that takes extra time. And it's that extra time that you'll need to be able to get in and out without getting hit. Find that timing, and you may have a chance against someone who's bigger and has a longer reach.
Here's a wonderful example of half-beat timing:
Hope that helps, and good luck!
Can't add comments yet, but just wanted to say that Steve Weigand has the best answer here. just wanted to add that with time you should incorporate some "dirty boxing" into your arsenal when fighting bigger opponents (5ft5 person here). There's no other way of beating somebody who has been training as long and as good as you, and weighs 10kg+ more.
Block with your elbow strikes instead of normal karate blocks.
Block their kicks with your knees, or with knee strikes.
When coming out of the close quarter engagement/clinch, don't just move out. You have to "hook" your opponents ribs with your knuckle strike or better with a thumb strike to maximize damage to a single point.
All this is meant to do is to deter your opponent from strike you. You will start to notice some hesitation. So if you mix what Steve Weigand advised and what i explained here. You should be scoring 1st and 2nd places in no time ;)