The mfg of the crank might have that data but I've never seen it available elsewhere. I'd just mount the crank with any old BB you have and measure the chainline of the crank. From there it's easy to determine the BB spindle length you need. Just make sure not to mix ISO and JIS taper BB's.
And if you don't have an old BB lying around, you could mount the crank to your spindle and measure the distance from the bearing race to the chainring center and go from there. That sounds error-prone to me but could be done. I've heard of thin vs thick BB cups, though, so watch out for that.