yellowdingo, is this just an itch you're scratching, or are you going to be posting these regularly for a while? Will you eventually collect them in one place?
A one million room dungeon map really would take years to finish; something the size of Undermountain or Castle Greyhawk would be plenty large enough to qualify as a megadungeon (and would take plenty long enough).
Yellodingo, if you didn't want to go full-bore megadungeon right off, would you be interested in doing something the size of, say, The Temple of Elemental Evil or Thornkeep to begin with?