To be honest, length is not determinable by physical features, but girth is. I'll tell you what I notice that usually means "big one." Broad features. Not to be confused with "big hands, big feet good lord, big meat." If the guy's a thick necked and looks like he could cut a tree down with his teeth; chances are he's got a nice, thick slab between his legs.

The major thing that I look for is if the guy's got thick fingers. Thick fingers have, for me, been a 98% accurate sign that the guy's got a thick one.

Like I said, though. It may not be a long one, but it'll damn sure hit all the right spots.