I completely understand what you are trying to accomplish, Snarfblat. I myself have tried multiple attempts at replicating such horrible failures as:
1. Helm's deep, complete with the castle and 10000 itcthyosaurs.
2. The Dairy Queen where I used to work, down to the last condiment container.
3. My old high school, including openable lockers.
4. My house (which is possible in HL2)
5. Sea World...don't ask.
6. My hometown...fastest. crushed. dream. ever.
7. The U.S.S. Enterprise, with slight design deviations.
8. The tower of London (the entire thing).
9. The Cathedral of Chartres (my most recent flop) which takes about 35 minutes just to load the outer walls with no details added to them.
Notice a recurring trend? Big = disaster. Now, don't go getting discouraged (I'll leave that to Hammer). My suggestion is to make skillful use of custom textures (i.e, have a photograph of the area you can't afford to model on a brush JUST beyond a door at the edge of your level) and clever level changes so that you switch between levels in a way that seems natural. I call it Potemkin modelling (you'll have to research that if you want to truly understand its significance): you're making an area that looks out at another area that looks like its there except it's a "painting" (a texture), but you actually can walk there by going to the level change that takes you to your real version of it. Once there, you look back at where you came and see a "painting" version of it, again using textures. It's tricky, but you can break the Theatre into sections that way.
Got all that? I know it seems confusing, but in practice it works (although it does take a while). Happy mapping!