Crank length

It's all to do with leg length, so manufactures, assume as a rider gets taller, their legs get longer, therefore increase the length of the crank arms (from the bottom bracket, to the pedal spindle). And vice versa, shorter riders get shorter cranks.
The only way to see if it is a major problem for yourself is to go and test ride the said machine.

Hope this helps,

Rob
@we8ster

http://www.stevehoggbikefitting.com/blog/2011/06/crank-length-which-one/

There's never been a definitive conclusion on what difference crank length makes. For example, Graeme Obree reckons 9.5% of your height is a good guide for a crank length. If you are 5'10'' - 11'' this makes 170cm about right which as a general rule is shorter than perceived wisdom. The important thing is that if you have more than one bike that they are the same.

the local club reckon train on short cranks, race on long...
Can't comment whether that makes a difference, but I've got 3 bikes with 170 and the race bike with 175 and I have a different 'natural' cadence on each bike.

all the road bikes are 172.5 and my mtb runs 175 - have
to be honest and say I can't tell the difference !

If you have a substantial leg length difference then experimenting with crank lengths can be very beneficial, especially if the difference in length is in the femur. After a nasty leg break resulting from a motorbike accident I now use a 165 left crank and a 175 right crank, this has greatly improved my position on the bike.

Thanks for all the comments. I think it has allayed my fears about buying an off the peg bike with 170 cranks. Where I was worried about it affecting my pedaling it seems it might actually improve it!