“Is it possible, for example, to run a map the size of the United States on Unreal Engine?”
Yes. You have to use certain tricks to make that happen.
Remember, most video games are illusions. Normal maps make low poly meshes look high poly. Occlusion culling only shows the player what he’s looking at and hide the rest to save memory(or something like that.)
Most likely, you will use Steaming levels to load certain chunks of game depending on where the player is.
The real question is WHY would you do that? A game that size would be full of empty space, which is boring.
Personally, I’d take a smaller, busier map of Yakuza game over the huge, empty map of GTA5.
Edit: I don’t think the size of the team is much of a factor for UE. There are many modules that make automating processes simple, i.e footsteps. You can take a single footstep sound, modulate it and randomize it via audio cue to create several sounds that would have taken longer in another app.