How Mega is Mega ?

Just how big should a megadungeon be? How "mega" is "MEGA" really?

Mapping is never the problem, I love mapping, I could map all day and all night. Filling the map so people have fun is the issue.  So how mega should a megadungeon be?

A dungeon level I'm working on.

If you want the dungeon to be a dungeon wilderness it should be a big place. So big mapping for players is essential. Big enough to support many sessions of play. Big enough to have room to support the campaign if it's all the players want to get involved with. Big enough to cram a bunch of different monsters in together and not have them exterminate each other quickly. How big is a dungeon that supports all that?