This is a discussion, everyone is welcome if not rudeness, trolling or flood!
In my game, I have three landscapes - complex one 8x8 km with many painting layers, 3D grass, mountains, three small rivers and a small lake (all the water takes probably not more than 0.2 km²), medium complexity 8x4 km with two painting layers (still five are present in the material but not used) and 3D grass, but without relief and water, and pretty simple 8x8 km without anything - and at the editor start the computer freezes for 5-7 seconds during which VRAM stands near the maximum, and sometimes the editor crashes after this. I have 24 GB of VRAM which is the maximum for the gaming GPUs at this moment. So, which computer is good enough, for example, for the open world 100x100 km with ocean and hundreds of rivers, lakes, islands, hills and mountains? How much VRAM it needs and which GPUs should it contain? @SupportiveEntity, you are invited as the main knowledge-keeper about the UE, maybe you know this better than average UE user?
First of all: Have as much RAM in your system as possible. I have 64 GB, and that is already filled to the brim with smaller environments (8 x 8 km). If you can hold 128, or more, it would help. What you also need is a lot of page file space, preferably on other disk(s). I have 2 page files, both on SSD, each 524 GB - so 1 Tb in all - on other disks than my projects. But the amount depends on the amount of internal memory and the total size of your project or level. Make them fixed size on an newly initialized disk, so the OS doesn’t have to do rescaling, to prevent fragmentation.
It would help if the derived data cache would be on a different disk than your project, especially if you use HDD, not SSD, to limit the number of disk accesses, but I haven’t found a way (yet) to place it elsewhere.
I also found and learned that the size an d resolution of textures plays a part. I have a max of 24Gb in my graphics card too, and the only thing that will help here is NOT using the highest quality (4 or 8K) for all your textures; in many cases, 2K will do, perhaps even 1K. Also, the size of the texture files seem to matter because what I understood, materials are handled by the GPU, and the GPU memory for textures is limited, it will hold more smaller files than big ones, and so can handle more materials.
BTW: I’m using Windows 11Pro, but the principles for page file(s) are the same for any OS: preferably on separate drive(s) than system or other heavily used ones
Hope this helps
Hello @WillemGr! This description looks close to supercomputer. Won’t there happen so that the average user with the typical computer won’t be able to normally play such game?
Are you developing games, then you definitely need bigger iron, or work with just a subset of your worlds - use World Partition and load only those regions you are working on. But even then, the more memory, CPU and GPU power and size (and number) of memory and page files matter. If you are in a team, it might well be reasonable to use a lighter machine but even then, my comments apply, it largely depends on the workload of the project and the workflow.
For the player, a smaller machine may well be suitable for this kind of games, given the size of the landscape, but I have no reference. My guess is that a graphical card is to be preferred, as well as sufficient memory and CPU power, and certainly if a GPU is not available.
You missed the most important part imo… What are your expectations visually???.. Electric Dreams quality? Can’t help you there… But even an ordinary GTX1080 GPU w/ 16GB ram and 500GB SSD (to host 100GB swapfile) is possible… If you stick with using 4.27 or downgrade from lumen / nanite and avoid landscapes altogether (that’s really the most crucial part). You can easily go out to 200 x 200 km. And beyond (think 1000’s of km’s). But you may need to be a patient optimizer or performance wiz to get really good frame rates. That’s often the case anyway with UE, so its definitely possible… Good luck. 
Unfortunately, I cannot understand this phrase. Did you mean a 1984 film directed by Steven Barron? Or UK video game publisher, also from 1980s? Or something else?
In any case, from the game developing in 2020s I expect, of course, visual quality that is actual in 2020s. I think this goes without saying, doesn’t it?
If you are doing so, why are you using UE? Write in the pure C++, there are no Lumen, Nanite, landscapes and other crucial parts of the modern game engine there. But I guess you have never tried this, or you have nothing to do on the forum discussing UE. In any case, the question was for the UE and its features and not for the assembler or “high-level assembler” such as C++.
Will try to explain… Electric Dreams is Epic’s ultra-real graphics environment demo. Mentioned it as a talking point, so you can think about what you actually want, and what your own expectations are visually for your own games. There’s a lot more to this though… Read on…
For sure… But the ultimate end-results also come to down to the actual choices that you make, the pre-existing skills that you have, and what you opt to include or leave out. Crucially, it also involves having a stable editor that you can also be the most productive with. Along with hardware budget of course… But these are the Indie Forums (not AAA) and so nothing is fixed… So consider this… Better devs can often get more out of lower hardware and 4.27 than novices on the latest and greatest UE5.x 4090. See where this is actually going?
Suggestion:
How far along is the project? For anyone just starting out… While it sounds obvious, it can really help to just buy some nice looking packs with rivers and lakes, and see what gets you hooked, rather then worrying too much about hardware and PC (rig) specs right now. For others, who don’t have a PC yet, you can always look at pack / samples on YouTube, or follow some Indie work-in-progress threads…
Afterwards, decide on graphical visual goals, versus how much you can actually afford to spend on a rig today. If you still want the most ultra-real, you’ve got to go big on budget. A supercomputer??? … No… But ultra-realistic demands the latest & greatest… But always remember, lots of Indies are happily designing games that aren’t ultra, and yet still look great. So nothing is fixed. Also if you’re only starting out, it can take substantial time to build up a project to show off (year or two). By then graphics hardware options have improved.
Be aware too, UE Landscapes are very very resource heavy. So ask yourself… Do your games really need them?.. How about foliage? … Can the world be winter with dead trees. If landscapes / foliage aren’t critical, then use static meshes everywhere, as it’ll help save you buckets in hardware costs. As you can always ramp things up later anyway, when you know where the project is actually really going.
TLDR:
Explore Indie project samples and find something you like and then ask what hardware is needed for that. If your goal is AAA all the way, then you need a beast of a rig, and that costs 1000’s of dollars in most of the world… And nothing is really future-proof anymore. 
Thanks, I will try to see at the free time…
I am not a novice, I already develop the games in UE for almost 4 years.
It is now getting close to the alpha version.
The “rig” means not the skeleton rig of the character?
Of course, I can convert the landscape to a mesh, but this won’t consume less resources (if not significantly decimate in Blender), but placing the water will be much harder.
Again, I can place the grass and bushes in Blueprints, but this won’t change anything if not significantly reduce quality.
Not AAA but III.