I'm no expert, I just like exploring how things work (and I may be way off in left field). I drew some pictures to get an idea of what was going on. I left gravity out but it does play a role, especially if you're standing. I only looked at being seated. I presumed that there is an ideal leg extension power-wise that gets applied to the pedal spindle. You'd want that maximum power to be applied 90° to the line going from the pedal spindle axis to the BB axis (so that line is tangent to the circle described by the pedal spindle) . The distance from the pedal spindle to the BB axis is the crank length. You have two sides of a right-angled triangle and the third side is then the saddle to BB axis distance.
(casual position)

The leg extension for maximum power shouldn't vary, so you can rotate everything around crank.

(road position)

(recumbent)


I haven't looked at reach, but I imagine that there is some sort of ideal arm extension, then the angles between arm and torso and between torso and legs which would dictate the position of the grips (handlebar shape being pretty much irrelevant). Gravity probably has more effect since rider weight is totally shared between saddle and grips (if not pedaling) and the rider's center of gravity is also rotating around the BB axis.