I'm in the process of designing a Megadungeon, and it seems like people run large dungeons like this using the theater of the mind to avoid giving the layout away and allow players to explore, get lost, do their own mapping, and etc, as well as to utilize techniques like getting loose directions from npc's or giving the players mapscraps. (I've also read some valuable stuff on being sure to give the players good landmarks so they can orient themselves.)
But to me, good combat requires setting up interesting tactical situations, having a physical map, and keeping track of player and enemy positions. (I feel most people agree.)
So how do you balance keeping the players in the dark about the overall layout of a dungeon, while also providing maps of wherever they happen to be engaged in combat? (especially when you want to make an encounter that could bleed over into some surrounding hallways)
Additionally, how do you balance keeping players in the dark about the overall layout, while providing good information on localized geography? (Like if they're in a hallway lined with small rooms, you don't want to specify all the rooms by number while talking, that's just tedious, it feels like a thing that calls for being drawn out.)
Best Answer
A megadungeon is simply too large to feasibly represent during play at 1in=5ft scale, even if you were wanting to, without a lot of work. I find that drawing out every section either beforehand or during play in battle-map scale is a lot of work for very little value. It's more effective to save miniature-scale maps for where they are most effective.
So instead, represent only the sections where combat actually happens, and during the exploration segments of play rely on just description to convey the layout, keeping your large-scale DM's map only as a private reference. That description-only information preserves the sense of exploration and uncertainty, allowing the players to use/need their smarts to figure out layout secrets, and maintaining the possibility of the PCs getting legitimately lost (as opposed to the kind of "story says you get lost now" approach that doesn't suit a megadungeon).
You can do this two ways. You can have "combat arenas" prepared in miniature scale in as much detail as you want, and lay those down when combat is either already broken out, or when it's obviously imminent even if initiative isn't yet rolled. The upside is you can put a lot of detail into these set-piece maps, but with the downside that for your prepared areas to be predictable, you have to build your whole megadungeon to neatly divide up into predictable combat arenas.
The second way is to draw out the map on the fly when combat occurs, on a vinyl battle-mat or small sheets side-by-side, using your master map for basic layout and your notes for room features that are relevant to an exciting combat. The upside is lots of flexibility in handling combats wherever and whenever they break out and go to, but the downside is that the maps won't be nearly as pretty.
You can also mix and match these methods. Say you decide to draw everything out, but you find an excellent ruined cult sanctuary tileset and want to use them—nothing is stopping you from busting out detailed tiles for one or more special rooms in the dungeon. Similarly, you can prepare all your combat-area maps beforehand in gorgeous detail, but fall back on sketching rooms and halls on a vinyl mat when combats occur in unexpected places or if they overflow outside of the prepared areas.
The thing these share in common is that you don't show your players perfect, miniature-scale maps for every foot of the dungeon, only "zoom in" when combat happens. That keeps the rest of the exploration in the mind's eye, where inexact knowledge of the layout can keep the "megadungeon mystique" alive for your players so it can enrich their experience of the place.