In school we learn about about people like "Alexander the Great" and "Ramses the Great" But what sets these people apart from others as "Great", is there some set definition or standard that a leader/conquerer has to live up to in order to earn the title of "great"?
Rama IX of Thailand is called "The Great" by some Thai people.
There's no really clear way for one to earn that title, other than having enough people refer to you that way (for whatever reason) and then having scholars record it.