I'm german and I never heard about this myth, lol. That's the funny thing about the internet. There was Carl Großmann, a serial killer, while Groß=tall and Mann=man, therefore maybe tall man. The funny things about myth is, that once a thought is started, other will pick it up and credit new features to it.That's the one!
Are you talking about Der GroBmann?
The german bogeyman is Der schwarze Mann (the black man, this refers not to the skin color, more to a man in the dark shadows etc.).