So I am making a cool Minecraft world, and I exported a world as an fbx, imported it, all fine. However, it is over 1km square! Will World Partition work with that single mesh? And if not, is there a way to somehow adaptive subdivide the world mesh so it can use less gpu/cpu?
Your question makes little sense.
1 mesh is one object / one actor. Also one draw call.
This could either mean great performance if you place it in persistent and dont hit a tris limit for the end device/specs you are publishing for.
Or an unviable end result if you exceed your tris on screen allotment…
This mesh is MASSIVE, approx. 1km x 1km in width. It has over 150 textures on it. Definitely pushing the tris limit! 
Is there any way to kinda divide it up into separate meshes inside Unreal Engine?
No. A mesh is a mesh. You have to work the model itself to split it into something which can be used…
How do I do this inside Unreal?
What does that mean?
You don’t do that in unreal. Unreal is not a modeling program. You have to remap textures ans UVs on top of cutting up the mesh.
And as far as something which can be used, well, that depends on a miriad of things:
From how the scene is going to be set up, to what the purpose is (game, vs cinematic, vs archviz).
As I mentioned originally your question makes little sense given the information you shared.
Cool. Thanks for the help. I’ll leave this unresolved for now.