Picking the right crank length

This is very much a black art, even more so than getting a bike that fits you right. Generally, mountain bikers should use a 175mm crank. It's the most popular size for a reason, it is correct for the "average" rider. Are you average though? A lot of downhillers and trials riders choose to run shorter cranks to gain ground clearance while keeping the centre of gravity low. The downside is a loss of power. A longer crank will give more power (due to the increased leverage), if your legs are upto it.

Summing up what is known about crank length (basically): the longer your leg the longer a crank you should run. Middleburn make cranks in 165/170/175/180mm lengths.