To understand crank length, you need to understand 1) angular velocity (loosely, "rpm") and 2) linear velocity (just plain speed over distance).
For a given angular velocity, a shorter crank makes your foot have to travel only a slower linear velocity, because it is making a smaller circle. For example, if you mount two wheel speed sensors on your fork, one near the hub and one near the tire, they will both register exactly the same speed. But the spoke magnet closer to the tire will be moving WAY faster than the magnet mounted near the hub.
So on level ground, spinning away at 90 rpm, your legs don't have to move as quickly with 165mm cranks as opposed to 175mm cranks.
The downside of short cranks is that you don't get the leverage on a climb. Or, on level ground, you can choose to dump the chain onto a bigger gear, spin more slowly, and maybe ride faster.
Now having said this, I will admit that I have been using a fixed gear with 165mm cranks for the past few years. I have completed PBP 2011, LEL 2013, and a California Triple Crown in 2012, all on the fixie. In my experience, when compared with riding similar distances on a geared bike (a 1000km ride nonstop in 2010), I find that I am much fresher on the fixed gear and 165's after the first 100 miles, and it takes me much less time after the ride to recover. I think that bigger gears, longer cranks, and lower cadence serve to load up the muscles with lactic acid.
I say this even though I'm often way overgeared on the hills with the fixie. The advantage of having gears is that you can shift down (and I mean REALLY shift down, like to 39x23 for a 6% grade) and quite often get up hills faster than trying to leverage up with long cranks and a bigger gear (which tends to tie up your legs more). I think there's a lot to be said for spinning up hills at close to 90 rpm, god knows I've been left in the dust by a lot of riders using this strategy! And this is much easier to do with shorter cranks.
So, conclusion: I don't think I'd go any longer than 170mm, even at 6'1" (me) for distance events. It might be different for short (40km) time trials, or hilly road races. But if you're doing ultramarathons, I think spinning is the way to go. You want to stay fresh as long as possible.
Luis