Hi, take a look at this here Landscape Technical Guide | Unreal Engine Documentation
AFAIK the max size you can have for a single landscape is smaller than 16384x16384 (you can ofc have several smaller landscapes that form a larger one). AFAIK this with the 20km is what you can do without world origin shifting, but I never build something that large, so no idea there.
Also you can’t have a size like 16384 for your landscape, UE needs a power of two per section and with that you won’t end up with something like 16384. In the link above are the recommented landscape sizes.