If you click here, that will link you to a web-page which outlines the German aristocratic system. From that, it seems that 'Herzog' - as Freddi says above - and 'Ritter' are the only two consisting of six letters.
I seem to remember being taught at school that the nobility in 18th century Prussia (a german state) were known as the 'Junkers'. Presumably, the singular term was Junker, which would be a six letter title