Although it's been covered quite a bit more technically, both on this forum & elsewhere on the web, I've always felt a good way to think about crank arm length might be to go back to a common childhood experience, at least a common experience for those of us who grew up with snowy winters...

When you were a kid, did you ever try to literally walk in your dad's (or mom's) footsteps, ahead of you in the snow? Their gait was much longer than yours, I'll warrant; and you likely had to almost jump to put one of your feet into one of their footprints. If you can remember such an experience, can you remember how your body had to shift in order to do this? How far you had to stretch your legs & hips to make those steps?

That's essentially what you're doing when you use cranks that are too long for your leg length.

Some people might get quite butch & venture the equation: longer cranks = more power; in fact, too long a crank = hip displacement + eventual knee damage.

The truth is there's no substitute for training and it's best done with an optimal fit.