So... being shorter doesn't matter. In fact, in this situation, it is an advantage.
I'm going to point out here that if you're not careful, your character that you want to have a badass moment is going to look like an complete asshole because they are humiliating another character who they actually are better than. If they haven't been trading off humiliations to build tension (safely so no one gets hurt) then you're going to have a lopsided “sparring” session. Or, you will, if you haven't been building up your narrative to support the humbling. (And there is a lot that goes into these scenes to make them work, which, if you're not familiar with how martial arts training works then you'll approach it from the wrong mindset and hit Starke's pet peeve.)
Let's hit the ground running with the high points:
Sparring is not play fighting.
Sparring is not so you can humiliate your opponent.
Sparring is not about fighting anyone or anything.
Sparring is a training exercise so that both of you can work on improving specific techniques.
Do people in real life get carried away sparring? Yes, they do. Are they complete competitive idiots about it? Yes, they are. Do they get punished for it? Absolutely, they do. This is especially true where they'll be expected to put their lives on the line. Treating violence like a game is detrimental, and leaves the trainee ill-prepared for dealing with real danger when in the field. Make no mistake, that is exactly the mindset you are describing in this question. Your characters (at least your minor ones, the trainers in this situation if your leads are too young or too dumb to comprehend their reality) should care about stomping this attitude out. And you as the author should to. Why? Because if you don't take the violence in your narrative seriously neither will your audience.
There are rules, as a writer it's imperative you establish the rules (and there no rules means you haven't established them for violence in your setting and therefore won't be able to establish the baseline that can be built upon later) and one of the rules is that you're not going to spar someone with a weapon (wood or otherwise) unless you both have weapons. There's not a lot of value in having one character spar with a weapon and one spar without one unless it's a knife, and the point is learning the dangers of knife fighting. And knives make sense because they are in the same distance range as fists. Swords are in a completely separate distance category. They are mid-range weapons.
You don't practice disarms by sparring, you practice disarms by practicing disarms in a controlled setting where you're repeating the motion over and over. Can you humiliate a person by being good at practice disarms? Yes. You do it by being a complete dick. It also requires the character in question to be better at the moves in question than the other character performing them because they need to be able to confidently, or at least believe they can, counter the other person's growing anger while taking the move a step further or two than they're supposed to. It also means they can get away with it without arousing the suspicions of their instructor (or act with their instructors approval) and no one gets hurt. (We hope.) For a character to do this is a sign of overconfidence, FYI. As is trying to humiliate someone in a sparring session. The characters that are good enough? They don't do it.
In fiction, good sparring scenes serve one real purpose. They establish a baseline of skill in safe setting so the audience becomes comfortable only for that to be disrupted and thrown into chaos when the characters encounter real violence. There's two paths for this. Either the character is a big winner only to be brutally beaten later, or they get dumped on their ass to find that they're actually much better prepared than they thought later when it matters. That's why so many stories with these scenes dump their MC on their ass, especially in any Wuxia or Shounen manga. This is because the authors of these stories understand that sparring has no reflection on how well a person will do when they're allowed out of the training safe space. If your baseline is: my character is awesome. Then it's all downhill from that point on.
Say it with me, Losses Create Tension. If your character is winning all the time, you have no tension and your fight scenes will be boring. The goal when it comes to creating a character who is good at fighting is to make other characters look better. Or, from an antagonistic/mentor point they exist to establish the height our MCs must eventually reach/how much further they have to go in their journey. Kakashi's fight with Zabuza or the first fight between Itachi and Sasuke in Naruto are both great examples of how to do this well.
I'm not saying you can't write a sparring scene like the one you intend. I can't tell you to do anything, what I want to you to start doing is considering the implications of the scene, what it may say about your world and characters that you didn't intend, and it's overall impact on the whole of your narrative. Narrative gratification here is work you'll need to do to build back your tension later. Is it a win your MC can afford?
Now, you can look at any martial art that has a ground fighting component for what you want to do unless you're planning on having the duelist spar with their sword. If you want that, you're going to have to do a lot more work with a smaller pool. This will be doubly true if your characters are of European descent and you want to avoid the East Asian martial art styles.
Type: “how do you knock your opponent off balance?” into Google and you'll find a lot of variations.
The basic concept behind putting someone on the ground isn't strength, it's balance. The key is disrupting your opponent's balance. If you're skilled enough or your opponent's footwork is bad enough, it can be done in a single move. In fact, it can be done a variety of different ways from a variety of different moves from countless different martial arts. The question isn't can it be done, the question is how does your character want to do it? The fact they are short only helps them because their center of gravity will be lower than their opponent's, they don't have to work as hard to maintain their balance, their stance doesn't have to be as deep, and they will have an easier time knocking a taller person over. Most people who've never practiced martial arts have no idea how foundational the footwork is or how important the feet are to staying upright.
I personally like reviewing Silat for studying balance, not necessarily for techniques, but because I find their instruction on the concept easy to grasp/digest. They do the string on the top of the head and the balance triangle, which if you can wrap your mind around that you'll be able to conceptualize fight scenes where the character focuses on knocking an opponent off balance better.
The above is a more advanced video, but if you have no martial arts background or even a sports background whatsoever then you want to aim for instructional videos that focus on concepts over techniques. The advice is always write what you know and if you don't know learn. Copying techniques onto the page won't create a great fight scene. Understanding the concept, philosophy, and basic body mechanics behind the techniques will get you much further. None of them are stand ins for real experience or doing it yourself. If you really want to be good at it, find a martial art you like, find a school nearby, and invest the time.
All of your characters' martial arts moves (whether they are dueling with a sword or fighting hand to hand) function around the body's center of gravity. Your center of gravity is slightly above your hips and in your core muscles i.e your abdominal muscles. They will be trained so maintaining their balance is second nature. When martial artists talk about overextending, they're talking about putting your weight past your balance point which puts you in danger of losing your balance/falling over or being grabbed, kicked, etc and getting thrown, tripped, or forced into a fall. This can happen when you're throwing a punch, doing a kick, lunging with your sword, or doing any other sort of movement. You end up in a position where your balance between your front and back leg is destabilized, which creates the opening for your opponent to throw you. Or when you fall over on the ice, because that happens too.
Some other martial arts to turn to:
Baguazhang (Seen in Avatar the Last Airbender as the basis for Air Bending)
Taekwondo (if you want to do it via kicks, all kicking martial arts innately focus on balance)
I also recommend dipping your toes into live action martial arts flicks to start getting yourself accustomed to more complex choreography. This is getting yourself out of the animated space (like in anime) and into the space where you have to watch a live person perform the techniques. Asian cinema has a different choreography style than the US does, because there are different cultural expectations. Overall, the choreography is more intricate and they break the action down a lot more (as opposed to American media where they zoom out to cover for the stunt double.) It's easier to see how the bodies are working and they put a lot more focus on destabilizing balance as part of the fight sequences. Hollywood doesn't get into the weird martial arts shit unless it's an actual martial arts action film. You can also do an Avatar the Last Airbender/Legend of Korra deep dive because the action there is also usually on point, but I'm a proponent of going to the source when you want to learn something. So, you know watch Alchemy of Souls instead.