They are not very long. "long" at best.
You know if you have one when you flex to have a pic.
Here are very long biceps :
Notice the left one
Very true! I remember when I first started reading BB mags about four years ago, Iron Man mag had a caption under a Sergio Oliva pic for a bicep training article. It stated something along the lines of Oliva built his long bicep "bellies" by performing his barbell curls with an "ultra" wide grip, and doing all curling exercises from full lockout to peak contraction.
I believe length and shape deals mainly with genetics.