There are some length options for the crank arms - like 170mm, 175mm.
What is the difference for the rider? In which way does it effect the commute?
Tried looking at Selecting the right crank length, didn't get a comprehensive answer.
If you keep the rest of the bike the same a shorter crank gives more ground clearance at the bottom of the stroke, and that's going to be the main thing most people notice. You're unlikely to notice a change in power output unless you're shorter than, say, 1.7m, which would put you firmly on the down-slope of the power curve (ie, the part where longer = less power). You may notice a change in comfort or pain levels due to the increased joint articulation from the longer cranks. IMO most people would be better off switching from 170mm cranks to 160mm or 165mm cranks.
At the high performance end the power output varies in interesting ways. Since power = torque x speed, and torque = force x radius, a longer crank means more torque but also more leg movement to produce it. And vice versa for shorter cranks. In practice most athletes have a fairly flat relationship between average power and crank length in the range 150-170mm, but it's also something that is little studied.
A study by Martin (2001) suggests that it doesn't really matter:
I found some thought experiments and this interesting summary/bibliography. (edit to add) Ian Sims at GreenSpeed has long had an interest but I can't access their site right now, so here's an A2B magazine post. It's worth noting that Ian has changed his position completely, based on evidence that he mentions in that article. Previously he was a fan of long cranks, now he's suggesting that short ones work better.
Edit by mattnz replacing Mσᶎ's link: PowerCranks have an interesting sales push with useful links and discussion. They also provide test results showing advantages of short cranks. Apart from being benificial to people with poor flexibilty and knee problems, short cranks provide major racing benefit from improved aerodynamic position. They also discuss why its a myth you need long cranks for climbing.
(moz again) My experience is that shorter than conventional cranks work better than I expected and reduced my knee pain somewhat when riding hard (long tours or fast commuting for 40+ km/day). I'm 1.8m tall and ride 155mm cranks because those are the shortest I could get without paying a price premium. My partner is 1.5m tall and prefers 145mm cranks. Going up to the 165mm cranks on my load bike feels like a significant jump, but I haven't tested them because I don't have access to a power meter.
It affects several factors:
The range of motion during the pedal-stroke
Time per revolution (as a result of the increased radius)
and the maximum torque a rider is able to put on the crank
The majority of riders only pay attention to range of motion. For taller people and those with longer legs, longer crank arms are necessary to generate a larger range of motion in the thigh/hip. This uses more muscles and feels less cramped. However, shorter riders might benefit (if they have the power) from longer crank arms for the sake of increased torque. However, torque is generally ignored since the increase is quite minimal (only heard of a few pros doing this). Most people will be fine in th 172.5mm range. If you are tall then I would say it is an absolute necessity to get 175mm crank arms (I myself am tall and feel cramped when riding anything shorter). Hope that helps.