Blue
Ravenous Bugblatter Beast of Traal
I know this is a pretty subjective question, but I was wondering if there was some kind of consensus... Clearly a 5 room dungeon is not mega, and a 1000 room one definitely is, but in between...
Is it merely a question of size? Are other characteristics important?
To me, a megadungeon is a sandbox that can (and needs be) adventured at a good range of levels.
Big enough to have distinct sections and themes. Choices about where to go so it's not linear (though you will need to get through some parts to get to others). And enough content to last for levels. Including that some places will repopulate, groups are dynamic, etc.