As always, I think there's a good sheldon brown article on the subject.
Your cranks are levers, so as I understand it a shorter crank lets you spin a little faster but makes it harder to exert force. A longer crank will mean your legs are travelling a bit further so it will slow your cadence down, but you can get a greater force on the pedals... or something like that.
I think bike fit comes into it a lot, some frames you may be suited to a different length crank - but that's just my uninformed opinion :)
I've chosen mostly to answer the latter, but the simple answer is (for adult men):
if you're: really short: 165mm, short: 170mm, average height 172.5mm, tall: 175mm, ridiculously tall 177.5/180mm. Might need to go up or down a category if your legs are particularly long or short compared to your torso.
The in-depth answer:
There are a lot of competing theories on crank length.
I haven't got time to go into it but some key points:
* some people say it is all about a formula based on leg length, find the inseam measurement, multiply by a constant and you get the ideal length, then you should try to get the closest available crank to that. If you google you will find many such formulas, most giving similar results.
* some people claim that almost everyone should use standard lengths between about 165mm and 175mm, and that the formula people are oversimplifying the biomechanics to a linear ratio
* google 'steve hogg', he has pdf's freely available with in depth discussions of a lot of these topics. He's open to the fact that having your 'ideal' length could be an advantage but he points out that going too long could put too much strain on knees and ankles
* using cranks that are too short will facilitate high cadences but taller people may feel they have a restricted range of leg motion
* using cranks that are too long is like adding easier gears to your bike and slows your cadence.
* apparently almost no tour pros use a crank length longer than 175mm in a road race, even the super tall ones. It is however very common to use them in a time trial (or in MTB).
I'm quite long legged (94cm inseam) and i've recently experimented with a longer than standard crank. Depending on different charts my 'ideal' length is between 180mm and 205mm. I've generally used a 175mm crank arm, but i splurged on a dura ace 177.5mm (only Dura Ace, Record and TA tend to have longer sizes). Any longer than 177.5 and you need a special bike or your pedals will scrape the ground!
My impressions of longer cranks:
* at first the wider circles you need to make with your feet seem like more effort, but this sensation fades once you have done a few rides.
* it is harder to ride standing on the pedals, you get used to it, but it is not as fluid as with a shorter crank. Acceleration is slightly impaired. I think this is probably why tall pros don't use longer cranks.
* at first it feels like your feet are too close to the ground and that you will scrape going around corners. However even when riding criterium races and going very low around corners it has never happened and i've never felt compromised for ground clearance in that situation.
* because the longer cranks will extend the pedalling circle in each direction you have to move your seat forward and down to preserve the maximum leg extension point of the pedal stroke
* as a long-legged person, moving the seat forward was great for my riding position. Frames tend to have top tubes as long as seat tubes and i'd actually prefer a top tube a few cm shorter (my bike is 60cm). The longer cranks slighlty reduce the effective top tube length so i am more comfortable with the position of the bars, and i'm slightly more aerodynamic (2.5mm!), and have a lower centre of gravity (good for cornering).
I originally wanted longer cranks because I didn't feel my leg was bent enough at the top of the pedal stroke and that i wasn't utilizing my quads or hip flexors much. I felt like i wanted my feet to be doing bigger circles. The 177.5mm crank has solved those issues, and i'm generally happy with the setup, but after about 4 months i'm still not sure its any better than 175mm. Not because i can't feel a difference - it's a marked difference, just that across the balance of different riding situations it's not always better. If i was time-trialling or doing triathlons or touring I'd definitely recommend longer (although who's going to put DA on a tourer?). But otherwise, probably not worth the expense and complication for most people.
I'm not sure whether or not it's a myth regarding Indurain. I might be thinking of another rider but I thought I heard that even Indurain had only used the famously long cranks in time trials, not in normal road stages. Would be cool if someone knew whether this was true?
Indurain apparently used a 190mm to break the world 1-hour record. Below is a list of crank lengths for famous cyclists throughout history. I'm not sure how accurate it is given that I'm 80% sure armstrong didn't use 175 (maybe in a TT). I had heard that he actually used a 165mm crank, with a stamp saying 170mm for some reason - macho factor?
I would also be surprised to hear that Robbie McEwen uses a 175.
Jacques Anquetil 175mm
Lance Armstrong 175mm
Magnus Backstedt 177.5mm
Chris Boardman 170mm
Santiago Botero 172.5mm
Angel Casero 175mm
Mario Cipollini 172.5mm
Fausto Coppi 171mm
Malcolm Elliott 172.5mm
Tyler Hamilton 172.5mm
Bernard Hinault 172.5mm
Miguel Indurian 180mm (190mm for second Hour record!)
Laurent Jalabert 172.5mm
Greg Lemond 175mm,
Brad McGee 175mm
Robbie McEwen 175mm
Eddy Merckx 175mm
David Millar 175mm (180mm in TT)
Francesco Moser 175mm
Marty Northstein 167.5mm in Keirin (170mm in kilo)
Graham Obree 175mm
Marco Pantani 170mm (180mm in mountains)
David Rebellin 172.5mm
Roger Riviere 175mm
Jean Robic 170mm
Tony Rominger 172.5mm (175mm for Hour record)
Oscar Sevilla 175mm
Jan Ullrich 177.5mm
Rik Verbrugghe 175mm
Erik Zabel 172.5mm
Alex Zulle 175mm (180mm in mountains)
Alberto Contador 172.5
Fabien Cancellara 177.5
Tom Boonen 177.5mm
Allan Davis 172.5mm
Gord Fraser 172.5mm
Oscar Freire 172.5mm
Thor Hushovd 175mm
Giovanni Lombardi 172.5mm
Alessandro Petacchi 175mm
Fred Rodriguez 175mm
Erik Zabel 172.5mm
Thanks for a long reply -- so you feel that 2.5 mm makes a marked difference? I mean, all the points you've listed is what you've felt when you switched from 175 to 177.5. For example, to compensate for a different crank length, you would need to lower the seat by 2.5 mm, isn't it (in fact, even less if you move it forward)? Have you lowered it like that and there has been a difference? Or you changed it much more? Is it then a difference between two saddle positions?
I am asking because my cranks a just a little longer than recommended, I know I will have knee problems in the future, so I would have liked to switch to slightly shorter ones, but I just can't believe 2.5 mm could make a difference. Hence, all the questions :-)