Creating a large landscape

Hello, I didn’t know where to write, so I’m writing here, OK, I’m trying to create a landscape using world composition, using elevation maps, I’ve studied the information Landscape Technical Guide | Unreal Engine Documentation and I still haven’t figured out ,I need a landscape of about 200 km by 200 km,what size of elevation map should I use, and how many parts should I cut it into.

LOvely! This is very helpful for starters like me.