So if one level of our MEGACUBE is 26x26 one page dungeons then one level would take 676 pages to map.
and if it's a cube and there are 26 levels=17,576 pages!
plus a one page key fore each level and one for the surface, that's 17,603 pages.
That's one hell of a DM binder.
Of course, there would not have to be a one page dungeon page for every 300'x300' section. Maybe some areas are empty (solid earth) or simple tunnels/passages that link more densely populated areas. These would need no more than a line or two to be indicated on the level key.
In the past I have not been the kind of DM who would be attracted to this level of detail at all. As a DM or as a player I've rarely even used graphpaper. But I see some exciting possibilities presented in having such a large three dimensional area mapped so precisely.
I usually handwave the precise distances/relationships between areas, levels or subsections of a dungeon. Who cares exactly how long a particular tunnel or flight of stairs is, that's what I say. But what if the relationships between a room in a dungeon and other rooms on all sides, above and below it is really important? Say, if a person could walk through walls (or dig, melt, explode or otherwise tunnel through stone/earth).
The article linked above describes a tactic employed by the Israeli army. Because of IEDs and snipers the streets were so dangerous as to be unpassable so they began tunneling through the walls and floors of buildings.
Here's a quote from the General who developed the tactic:
“This space that you look at, this room that you look at, is nothing but your interpretation of it. Now, you can stretch the boundaries of your interpretation, but not in an unlimited fashion, after all, it must be bound by physics, as it contains buildings and alleys. The question is: how do you interpret the alley? Do you interpret the alley as a place, like every architect and every town planner does, to walk through, or do you interpret the alley as a place forbidden to walk through? This depends only on interpretation. We interpreted the alley as a place forbidden to walk through, and the door as a place forbidden to pass through, and the window as a place forbidden to look through, because a weapon awaits us in the alley, and a booby trap awaits us behind the doors. This is because the enemy interprets space in a traditional, classical manner, and I do not want to obey this interpretation and fall into his traps. Not only do I not want to fall into his traps, I want to surprise him! This is the essence of war. I need to win. I need to emerge from an unexpected place. And this is what we tried to do.”
“This is why that we opted for the methodology of walking through walls. […] Like a worm that eats its way forward, emerging at points and then disappearing. We were thus moving from the interior of homes to their exterior in a surprising manner and in places we were not expected, arriving from behind and hitting the enemy that awaited us behind a corner. […] I said to my troops, “Friends! This is not a matter of your choice! There is no other way of moving! If until now you were used to move along roads and sidewalks, forget it! From now on we all walk through walls!”
I read that, and it makes me think of goblins invading an underground dwarven fortress, or adventurers digging a tunnel to bypass all the traps set up around and evil temple.