According to Peter White your crank length should be 18.5% of the length of your leg measuring from the top of your femur to the floor.
Apparently, I may have misread the op's post. Anyway, if the length is not stamped into the crankarm, simply grab a ruler and measure it. Crankarm length is measured in millimeters from the center of the pedal shaft to the center of the bb spindle. If your ruler doesn't have metric graduations, the conversion factor for inches to mm's is 25.4.
That's what I thought at first, but after reading the post a second time, I'm not sure because of the wording. Either way, I think all the bases have been covered.
@hophead Thanks a lot for the picture, I've just found this thread through search and I found the answer thanks to you.