Crankset Arm length

crank arms come basically in three sizes 170mm, 172.5mm, 175mm. The size you need is based upon your physique - general rule: the shorter you are the shorter your legs (in this case femur length is most relevant) thus a shorter crank arm and taller = longer.

you could adjust your seat higher/ lower but realize that the length will effect your pedal stroke in the east west axis as well as the north south. example: imagine a crank arm of 500mm - you would be kicking so far forward on the pedal stroke you would lose a tremendous amount of power. The idea is to get a match to were you are dropping you foot down straight down maximizing the power generated.

A professional bike fitting at your local bike shop.  This isn't a snarky response, it's the reality of human anatomy.  I'm 6'4" and have short legs (32" inseam).  I know 6'4" guys who wear 36" inseam.  A professional bike fitting will make you more efficient, more comfortable, and may reduce potential injuries.

A long legged 6 footer would probably want a 175cm crank.  If you're long torsoed, then perhaps a 172.5. 

I believe that all the studies indicate very little performance difference between the lengths.  basically- it doesn't matter all that much, so don't worry if your bike comes with one or the other.

This^^^^.  The current trend is to go shorter to open up the hip angle but like previously mentioned, studies show little performance differences.  I'm 6'0 and run 172.5.  I don't notice any real difference between it and the 175 that I used to run.

172.5 or 175... there isn't huge difference between the lengths... its like 3/32" (not much). 172.5 is pretty standard for 54-58 size bikes. 170's are usually found on 54 or shorter and 175's on 58 or larger.

On my bike is a 175 and I am 6ft tall... but I do have longer femurs than average. It boils down to preference and comfort.

I would recommend 172.5, kinda hard to go wrong and is pretty standard to 6ft tall riders.

John Cobb recommends the following for triathletes.

4'11" - 5'1" = 145mm cranks 

5'2" - 5'6" = 155mm 

5'7' - 6'1" = 165mm 

6'2" - 6'6" = 170mm 

There is speculation of power with the lengths as well.  Lance Armstrong pre TDF win and during TDF reign shortened his crankset by 5mm to adjust for the change is style from power to cadence. 

Do you have a link that elaborates on this? Interesting numbers there.

http://www.endurancecorner.com/John_Cobb/Part4

http://forum.slowtwitch.com/cgi-bin/gforum.cgi?post=3440512#3440512

http://mountainbikingzane.files.wordpress.com/2009/09/cranklength-1.pdf

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