I've played around with this some, using the three most common sizes, on the same different bikes, over a longish period of time. For me, 6' 1" with a 34" inseam, it was like this:

- I used Ultegra 6500 double cranksets with 53-39 rings and the same Dura Ace 7700 109.5 bottom bracket the whole time. I used an 11-23 cassette the whole time.
- I had 170s first. They, at the time, felt very fast, but I found my self mashing a lot (I was out of shape and had a bad fit).
- I tried some 175s for about six months. They worked better for a while and felt a little easier. ...Then, I started spinning more often.
- I got another bike that was equipped with 170s and decided to use them on the other bike. If felt like I could spin more smoothly, and easier than I could with the 175s
- Soon, I sold that crankset and got another for the new bike that was 172.5. It seemed that my cadence wasn't as smooth now, but still quite smooth.
- During this time, I still had the set of 175s that I put back on the old bike. When making a direct comparison of the two near identical setups, I found that it was the crank that I liked more ...but it was same model crankset, just a different size(?)
- I try another 172.5 on that bike, and pretty much instantly, I find it to mesh better with my riding style and cadence.
- Testing out other 170 cranks on other bikes has led me to believe over time that it is my ideal size of crank arm. The effect is almost intangible, but I plan to replace all my cranksets with ones that have 170mm arms at some point.


Keep in mind that I did this 'experiment' over the past several years and it's intended to be an observation, at best.