Map Size?

[Vale]

So for the custom story I have oohhhh so ever slowly been working on, some of the maps will be quite large.
What I've been thinking about is making most of the game on one 'supermap' as one complete mansion/house kinda building, seamless and fully connected. The only thing is: an unfinished component from one (of 5, maybe 4) floors which will probably compose only a fifth of the floor has surpassed three megabytes.
The final map would likely go well beyond 50 mb if it was all integrated.
So other than loading times and possible layout issues, would the game lag to an extreme level? Or would the occulusion take care of the size worries for me?
I mostly want the map to be one whole to make it feel like a complete environment, and IMO the layout of the mansion would be easier to design and come up with if its all in one map. Do you guys have any experience with a map of this size?
Right now the size is theoretical, and people with outdated/shitty PCs are out of the question as my one 3 mb causes verrry minor framerate loss in same positions.
Just as comparison, my PC had no lag at all in any other Amnesia CS or even in our many hands 2 project.
Thoughts?