Where did you see the phrase "long man?" It might be helpful to know the context.
Like everyone here, I agree that "tall man" is the normal, everyday way to describe a man who has "great vertical stature." :)
However, in East Sussex, England, there is a figure "drawn" into the hill (by natural forces?) that looks like giant man. This figure is known as the "Long Man" in England.
Secondly, the American author Steve Englehart wrote a novel called "The Long Man," and in this case, it is not because of the protagonist's height. It is because of his longevity (the man is a super hero who does not age), so the author called his character the "long man."