I was obsessed with crank length a few years ago, and still have all lengths ranging from 165 to 177.5, and even 180s at one stage, (I'm 6ft with an 89cm inside leg) and, in my opinion, go for 165s, specially if you have anterior or lateral knee problems. My 'regular' road length is 172.5, and I sometimes race on 177.5 when I suspect I'll have to do a of off-the-saddle mashing.

This is a little off track, but this is how I came to use some 165s for a few months this year. There's apparently a trend at the moment in the U.S. tri community of using shorter cranks, because riders can get more aero. John Cobb says that everyone he's tested in wind wunnels on 165s has tested more aero, even tall guys. So, I bought a couple of sets of 165s and a set of 167.5s on Ebay to experiment with trying to get very aero.

Anyway, despite having long-ish legs, I felt fine on 165s (I suppose I did use them on the velodrome a long time ago), and had some good rides in the local group smash-fest on flat roads. A few months after this, I had a sore knee, and found the 165s very good to roll around on for a while.

If you wanna try different lengths, you should be able to get something cheap on Ebay