Reach is standing like this and measuring finger tip to finger tip.
http://content.draftexpress.com/gallery/RudyGobert/1339445253.jpg
Guys with wide backs and broad shoulders have good reaches but there arms are actually quite short.
Interesting, I remember reading somewhere that the measurement of reach is a pretty consistent indicator of athletic ability. Meaning that height being equal (or similar) the athlete with the longer reach is usually the individual with better athletic prowess (strength, speed, anaerobic capacity, etc). Can't measure heart of course but you get the point.
Measuring arm length is totally redundant. Boxers don't punch with their arms they punch with their shoulders. They tend to lean their shoulders into their punches, so you get a better measurement by measuring from the hands and across the shoulders. Boxers are always leaning in or they reach further using their bodies. Arm length measuring only works if the boxers keep their upper bodies standing totally still and flick out little arm punches, and that just doesn't happen. For an orthodox fighter they hold their jab arm out front with their left upper body toward toward their opponent. The body definitely adds more to how far you can reach.
When they would give you the pre-fight measurements on a boxing telecast the "reach" always measured wingspan.
But HBO innovated the idea of measuring arm length recently. Which is measured from the armpit to the end of the fist.