On a more serious note, I think torque is a big issue, which is why MTBs and tourers tend (IME) to offer longer cranks---getting all that mass moving, or doing so on rough surfaces. other than that there is an issue of comfort ... I have relatively long legs. I used to feel more comfortable with 175s. Now I am older, weaker, and used to 170s, and my 175s feel a little long ... but i doubt I would notice after half-an-hour.