We usually go with a 28" stick, but other arts(like Pekiti Tirsia?) hold the arm out to the side, with the fingers pointing down, and measure out from the armpit to the palm. I believe this is done to allow the maneuverability to allow the stick to be spun "thru" the armpit. The idea is to get maximum reach, but not so big that it gets in the way.
~Kev