ok, first thank you for your advice!
-I'm building a frame up Cross check 56cm for touring and towny bike. although i haven't yet bought the frame i have been sized on a 56cm CC. i have a fist of seatpost prutuding from the frame and the seat to handle bars is level. Which i like. BUT WHAT IS THE RECOMMENDED LENGTH FOR TOURING BIKES? 170-175 WHAT CRANKS WOULD YOU RECOMMEND? LOOKING FOR TRIPLE, STIFF, AND DURABLE
Generally tall people use 175mm and shorties get 165mm cranks. If you're in between, start with 170. If you have an average cadence, that's it. If you like to spin fast, you may consider adjusting to a slightly shorter crank. Likewise, if your cadence is slow, a slightly longer crank is warrented. Just keep in mind that +/-5mm doesn't make that much difference.
Things to consider:
1) make sure your crank matches the bottom bracket. Isis vs. square taper vs. whatever. This sounds obvious but consider how embarassed you'd be if you started to put it together and found you had incompatable parts.
2) Choose the bolt circle diameter (mountain, compact or road) so that you have a good choice of chainring sizes to match your riding style with your rear cluster.
3) Choose the proper bottom bracket length. I wont get into discussing chainline. Here's a good explanation. http://www.sheldonbrown.com/gloss_ch.html#chainline
I use a Sugino XD series crank. Reasonable quality at a reasonable price.
Stop now before you drive yourself crazy! Pick a random number between 165 and 175. Choose that crank length.
succumbs to errata
Remember that the shorter the crank, the less your knee has to flex through each rotation, this may be important if you have knee issues.
I was having minor knee discomfort on longer (50+ mile) rides using 175's. I switched to 170's (Sugino XD's) and couldn't be happier. Also keep in mind that if you choose a shorter crank, you may want to adjust your gearing to compensate.
If you want to be obsessive about it, google around. You will find plenty of rants on optimal crank lengths, historical data about the crank lengths preferred by winners of the Tour de France, formulas for determining the ideal crank length, and so on, ad nauseum.
Or you can just take Speedo's sage advise, or apply rwp's sensible rule-of-thumb, make your choice, and never give the subject another thought!