crank arm sizes are length. 180 seems too long. I'ld try 165-170mm. But u need to make sure ur bottom bracket can take the crank you are using. Most crank arms will suggest a bottom bracket that goes with their cranks.

As far as gear ratios go, just figure out whichever u like to stick with on ur current bike. Also take your terrain and the distance u are looking to bike into consideration, if u are gonna be hulling heavy things on ur bike, etc.

read:
http://www.sheldonbrown.com/singlespeed.html