Baked lighting for large scenes is hard, as Darthviper107 has said.
I have managed to bake lighting for a scene around 1km^2, but it took 2-3 rooms worth of computers at my university hooked up to the swarm agent to get it to work (we’re talking ~40 i5 4690k CPUs each with 16-32GB of RAM), and even then it took an unreasonable amount of time (several hours for a lighting bake with 1 bounce - not ideal).
It works, but the Swarm Agent stalls for most of it (if i remember correctly, it stalled at ~1-5%, then would suddenly be ‘finished’ a couple of hours later, then would take a further ~30 minutes to import it all back in to the host machine, all while being mostly unresponsive).
I would still recommend baked lighting over real-time (especially for VR), and if you wanted to go with the baked lighting method your best bet would be to break the scene up in to smaller levels and bake them out separately, then use World Composition to assemble your world, much like a traditional puzzle.