I think I'm right in saying that 170mm is the "standard" length crank that bikes are sold with.
Having both 170mm and 172.5mm cranksets, I did a fair bit of research recently to help me decide which bike should get which cranks: one bike is for distance and the other for speed.
The "common sense" opinion is that longer legs need longer cranks (and vice versa), which at first seems obvious. However, the more I thought about it, the less convinced I was.
When pedalling your leg, basically, moves up and down and you describe a circle with your foot. This size of the circle is dependant on the length of the crank but is unaffected by the length of the leg: a long leg will describe the same circle a short one.
Various formulas are offered in support of the "long leg, long crank" theory, but if you already know the answer you want then devising a formula to support it is straight forward.
However, you only need a basic understanding of physics to know that the length of a lever (in this case the crank) will affect the force required to achieve a set outcome (in this case turn the chainwheel once): a shorter lever will require more force than a long one as it is closer to the fulcrum.
Looking at it from another angle: a longer lever will achieve more from one rotation than a short one.
Other considerations are ground clearance (which is why shorter cranks are usual on track bikes) and the stress on your knees (a longer crank will flex your knee through a greater range).
So what did I decide? Because it will produce the maximum output and because I ride it less often (being aware of the greater stress on my knees) I put the longer cranks on the "fast" bike and the shorter on the "distance" bike.
Having said all that, I didn't set out to buy different length cranks (the 172.5mm set were just a good price) and wouldn't go out of my way to do so in the future: the affect is marginal at best.
170mm cranks will suit 90% of people 90% of the time [N.B. these figures are plucked out of the air and have no basis in fact].