My ring finger is a little bit longer than my index finger.
I am not inclined to fully believe this myth, but If I see a guy with a short ring finger I will use probability to assume that he may have a smaller dick than the next guy.
Honestly, I believe there is some sort of co-relation between penis and other body parts. For example, I am long and lanky. My dick started out as long and thin. Its just part of my natural body structure so it makes sense for it to be that way. A tall skinny guy is more likely to have a long thin dick than a short fat one.