What you’ll want to do in the general sense is have a few low resolution meshes for viewing at a distance, and then even smaller but highly detailed maps that are loaded when you’re nearby. That’s how Skyrim does it, though it’s far lower in size than what you’re suggesting.
Unreal’s scale is 1 unit = 1cm. That’s pretty small, but I believe using a different scale would mess up physics and the like, so you definitely want to try to keep with that. Also the general rule of thumb for textures is that one texture tile should cover 1 meter (100cm) but obviously that one’s broken a lot more often, your low resolution meshes might have textures 100x or 1000x that big.
Really to offer any more suggestions, I’d have to know approximately what your goal is.
- Day Z style survival?
- ARMA III style combined assault, where some players are on foot, others are in ground vehicles like tanks and APCs, and others are in helicopters and planes?
- Flight Simulator?
- Driving Simulator like Spintires or Euro Truck Simulator?
- Skyrim style open world RPG?
- Grand Theft Auto/Batman Arkham X/Assassin’s Creed open world third person action game?
- Black and White style god game?
- ???
EDIT: Basically, where is the player’s camera, what are they going to be doing with the terrain, how are they going to be traversing the terrain, and what (approximately if you must) is the core gameplay?