Is there such thing as a martial arts disciplines or techniques that "suit" someone? Like if someone is physically on the small, weak side but has good reflexes and spatial judgment would they emphasize techniques that rely on accuracy (or hitting people where you can cause lots of pain without lots of strength)? Or is it less what you learn and more how you use it? Am I making sense? (If the answer to the first question is yes, what's a good discipline for the character in my example?)
You train your body to your style. In terms of physicality, there’s no barrier for entry. You adapt the techniques to your body as you train. It’s a common misconception that you need a certain body type to be able to fight, or to be good at it. Training takes care of the issue. The kind of physical training you engage in will mold your body. Practice, dedication, attention to detail, correction of errors, and time are all it takes.
There are martial disciplines that will “suit” someone, but those are psychological and philosophical in nature. Learning is faster when you desire to learn, and when the fighting style doesn’t counter your own goals. If you are mentally rejecting your training, then training will be almost impossible and produce poor results. A fast, brutal fighting style that focuses heavily on joint breaks will not suit a character with a gentle nature, who wishes to do as little damage as possible. Someone who wants a more inward focused and philosophical martial art will do better with Aikido and Tai Chi Chuan than they will with the sport focused Taekwondo.
The problem with your example is that it’s incredibly general and focuses on the character’s body rather than the character themselves. There is no good answer to it because the answer is, “all of them”.
Using physical strength as a metric for what kind of fighting your character can participate in or what martial arts they can learn is for stat based games like Dungeons & Dragons. You can take the abilities listed and apply it to any martial art you want. As I’ve said before many times, it’s better to work the other way around by finding your martial art then figuring out what you’re characters physical skills are going to look like as a result of their training. Trying to apply the combat style the other way around ultimately results in window dressing. Especially since, “all of them”.
All martial arts will hone and develop your character’s reflexes. So, the question is ultimately not that your character has good reflexes but rather, how were they developed?
You learn to judge distance through training exercises with your partner. All martial artists need spatial awareness.
You will learn accuracy by practicing your strikes on targets and then against live human partners.
Martial arts don’t rely on physical strength alone for damage, it’s cumulative and a balance of multiple factors that are all developed by training. Speed, accuracy, flexibility, momentum, endurance, learning where to hit and how to hit to achieve your desired results, your ability to move your body together, timing, these are what most people mistakenly refer to as, “physical strength”. Often, genuine effort and hard work are mistaken for natural gifts.
”Who is my character?”
“What do they do?”
“What do they want to be doing when fighting? Their philosophical outlook on the nature of combat? Their morals? What do they believe in?”
“What kind of fighting will they be involved in?”
“What kind of fight scenes do I feel comfortable writing?”
“What is my genre?”
What interests you and your character, who they are as a person, what you’re going to ask them to fight in your narrative, and, of course, how closely you want to hew to reality are what you should use to narrow down your search. After that, it’s gravy.