This does not sound very realistic.
How would you create the terrain, are you talking about real topological data of the United States, or just something empty of that size?
If you’re talking about getting the terrain of the US correct then you’re going to use heightmaps? Thought about disk space limitations?
Let’s say you do tiles of 4096x4096 and a pixel density of 1 meter/pixel
So, for every 8km x 8km that’s 4 heightmaps of 32 MB each, total 128 MB.
But you need that times 1,2 million? Hm… ok, you could perhaps get away with 150 Terabyte of just heightmaps. But you also need textures, wanna do the math?
Now, your next task is just to populate this world with props. I guess that’ll take no time at all.
If you want to do anything of this size you need to do it procedurally, and not store anything on disk.