The OP sounds like he is running 3 campaigns at once, which would be a trial for any DM. A campaign setting only needs be big enough to offer the required XP for every class offered to reach Name level, and a little beyond. That will likely be dozens of characters though as they die and restart and try different classes. So it isn't a calculable number. Growing the setting is a constant session prep task. (And Gary says you only need to cover about 3 levels to start anyways, so don't pregame prep larger than what you need.)
The Blackmoor / Greyhawk dungeons in the heyday were virtually the whole campaign. They didn't have to be, but that is how I would define a Megadungeon: "big enough to support an entire campaign's XP requirements within". That means all the diversity required to learn to play the whole game. Make 3 of these and we're doing more work than necessary. Provide a number of modules of differing heights and supports and we have later, more standard campaign setting.
I suggest balancing the forces on the map (this means everything on the map). Build support towns, villages, keeps, watchtowers, as well as cities and realms.
