Hi I want to create landscape 500 meteres, Can you explain me How I must set this :
I would start here and work backwards from this:
https://forums.unrealengine.com/development-discussion/content-creation/28153-calculating-exact-map-size
To understand the concept of vertices and overall size this is a very useful resource:
Remember 1 unit in Unreal Engine is equal to 1cm.
Hope this helps, good luck