You don’t know what you’re doing, while UE4 can certainly be used to make something that looks better than Skyrim does, you personally are not capable of doing so, it took millions of dollars to make Skyrim and you are just one person without any experience. You don’t know what issues arise when trying to make a large map. Also consider, what is the point of making a large map? If you can’t fill it with interesting things then there’s no point. When you make a smaller map you can spend more time making interesting places and paying particular attention to details.