Generally tall people use 175mm and shorties get 165mm cranks. If you're in between, start with 170. If you have an average cadence, that's it. If you like to spin fast, you may consider adjusting to a slightly shorter crank. Likewise, if your cadence is slow, a slightly longer crank is warrented. Just keep in mind that +/-5mm doesn't make that much difference.
Things to consider:
1) make sure your crank matches the bottom bracket. Isis vs. square taper vs. whatever. This sounds obvious but consider how embarassed you'd be if you started to put it together and found you had incompatable parts.
2) Choose the bolt circle diameter (mountain, compact or road) so that you have a good choice of chainring sizes to match your riding style with your rear cluster.
3) Choose the proper bottom bracket length. I wont get into discussing chainline. Here's a good explanation.
http://www.sheldonbrown.com/gloss_ch.html#chainline
I use a Sugino XD series crank. Reasonable quality at a reasonable price.