In the US, hunters and hunter seat eq and in some cases, dressage, do concern themselves with suitability. I have seen hunter judges penalize someone obviously too small (tiny child on a men's heavyweight hunter) or too large for their horse (heel, ankle or more below the barrel and visible from the other side of the horse) In equitation, you must have a horse whose barrel takes up your whole leg so you are secure (hs eq riders ride with their whole lower keg snug on the horse, this provides their security in two point) and can use the aids correctly. Also, a very tall or long torsoed rider will have difficulty balancing on a short coupled or short necked horse, and over fences the will invariably weight the front end.
Dressage is a little more forgiving, but you still need to be able to get your lower leg ON the horse when you need to - a rider making dramatic movements with their leg or picking up their heel to use it is distracting.
Much less important in the Western disciplines - they ride with the lowerbleg off the horse, don't get in two point and give cues mostly with their seat.
After all that, it's personal preference.
I still want a horse that takes up my whole leg, but as I age, I like 'em a little closer to the ground.