What is the crank length on a Shimano Deore XT? I have this bike:
http://www.trekbikes.com/int/en/bikes/mountain/cross_country/superfly/superfly_9_8/

Frame Size is 17.5 
Crank is: Shimano Deore XT, 38/24

I purchased a garmin edge. It asks for Crank length for the cadence business. 

How do I get my crank length?  Should I get out a tape and measure it?
For that frame size, most likely 170mm, but you can measure from the centre of the pedal spindle to the centre of the BB:



A popular online bike shop lists available crank lengths of 165, 170, 175, and 180. Presumably if you buy the bike "ready built" it will come with cranks chosen to match the frame size. But 165mm cranks are relatively rare outside of track / fixed gear circles. 170 and 175 are pretty ubiquitous.