The role of body type in deciding tokuiwaza
The shape, strengths, mobilities, and capabilities most certainly have an effect on which techniques are most appropriate to use and develop. However, it is difficult to determine the preferences of one's body before choosing.
One must also be alert to the body rejecting certain techniques due to a fixable physical pathology. That is, not being able to squat down to the ground is a good reason to be bad at seoinage, but should be fixed because all humans should be capable of squatting to the ground and all athletes should be capable of standing up from a squat with weight on their back. The process of fixing these problems likely lies outside the scope of judo practice, and might indicate a need for supplementary yoga or weightlifting or doctor's visits.
How to pick a tokuiwaza
I (and more convincingly, my coach) prefer the approach of teaching low-ranked (non-black-belt) judoka a wide variety of the basics and seeing which ones stick. Those basics might include osotogari, kouchigari, seoinage, haraigoshi, uchimata, and tai-otoshi. Other instructors might prefer a different list, for instance sumi-gaeshi, tomoe-nage, kosotogake, kataguruma, sotomakikomi, and koshiguruma. A wide variety of techniques can be fundamental, depending on the curriculum.
When I was having trouble finding a true tokuiwaza, my approach was to spend three to twelve months on a given technique before switching. Several times I would circle back to a technique I had already done a "cycle" of. So for instance, as I very roughly remember it:
- For the first year, I was all over the place
- For six months or so I worked on kouchigari
- For six months I worked on seoinage
- I spent a year hopping between tai-otoshi, seoinage, haraigoshi, and kouchigari
- I spent a few months on osotogari and osoto-otoshi
- I returned to kouchigari for a few months
- I focused on tai-otoshi for a few months
During this time a given technique would rise and fall in its efficacy--sometimes without working on it! Over time I found that left-side osoto-otoshi and an ambidextrous kouchigari were working well for me, and so I started drilling variations, set-ups, and complementary techniques for them. This worked okay; my judo is still thoroughly a beginner's.
This method ended up being a combination of following what worked, doing remedial work on weak points, and developing complementary techniques to what was working. That's the best answer I know for how to develop a tokuiwaza, however, for me it didn't really create the kind of singular powerhouse tokuiwaza that judo is most effective with.