How would I go about measurements by miles or feet, and would it even be possible to load 100 miles long by 35 miles wide land without crashing the editor or reaching some “hard limit”
Not possible with a single landscape. Look into World Composition.
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?
Found the World Composition Docs at World Composition User Guide | Unreal Engine Documentation looks promising
Veovis Muad’dib It will be “open world” 3rd person, cant get into many more details about the gameplay style
So basically every chunk of land needs to be assessable not just viewable in a distance
None of the terrain will be traveled in the air. so that leaves out planes. blimps, choppers, gliders or any mmo style “flying mounts”
Perhaps you misunderstood. I didn’t say some parts would be low poly and some would be high poly. I said ALL parts would be low poly and ALL parts would be high poly. You’re going to have to do every inch of the terrain at least twice if you want to have any reasonable hope of a world that size without loading screens.
3500 Square miles, no problem…faints 
Not knowing how to do it, and just starting to learn Houdini myself as part of my pipeline, i’d be looking for something, either (a Tool) or a programmed Batch process that could take a height-mapped image in single Square Miles… and convert each detailed Square mesh into it’s low poly twin. (If you can conceive of building it square mile by mile that is)
After looking through all the old Asset store plugins for Unity, the best solution might be “World Machine” which is a separate program altogether. It just might fit your needs. While I was writing this I decided to …Search lol. 
Once you build in world machine I imagine you can just import the created meshes into UE4 by an algorithm for your low and high poly meshes depending on distance etc…
World Machine in thirty seconds: Link
And for what you were asking for: Grid building 3500 Miles…
Hell of a lot faster than it could be done by hand. 
World Machine doesn’t seem very suited to the task though. I’d have recommended it for a flight sim or something like that, but a third person boots on the ground game? It’s not going to be pretty without some serious time invested in cleaning up the World Machine output by hand.
I haven’t seen an easy solution to this problem, the issue is for people who have planned something out beforehand, like myself. I’ve drawn out a map, in all three dimensions, made materials, meshes, and models. But once I reached the point of making the base landscape for my project I realized my problem with finding the size, I’ve done the math, my base land would be about 160,000 square miles. I considered looking at World Machine but I moved recently and there aren’t any cable lines to my house so I have absolutely no internet to download this in time. I’m actually sending this from McDonalds where I could find internet. If there is an easy solution.
Info on my project:
The land part is about 337.5 miles across and 337.5 miles high, the whole map is about 400x400 mi.
The land is shaped close to a oval with ocean around it.
I want it closer to an open world, but can divide it into a 4x4 loading grid or in half, but I don’t know how to divide it and keep it in one map.
My computer can easily handle a large file like the map would be. But I don’t know if it could handle rendering it with all the meshes ie. trees, foliage
There won’t be any animation until I get a beefier computer and move the file.
It has at least 12 tiles with different materials.
I make all models and most materials on Blender.
And I’m hurting on money, so expensive is out of the question.
If you decide to go with World Machine, you can follow my guide on this blog: https://unrealmethodscom.wordpress.com/
There’s a guide on everything from creating the level, to importing, to texturing, and to add details.
Veovis is correct. It’s going to take a hell of a time to render, but doing this by hand? No way I would spend my time with that. World Machine has some nice, easy to use components as well, to create realistic terrain.