Hey,

there are many ways.
Any large Terrains are chunkable (set chunks in SED, "terrain_chunk") and it s also possible to use LOD (terrain_lod).

An Terrain edior is not necessary.
Although MED has a size Limit but you can create multiple terrains.
There is a tool (found in an AUM) that can connect terrains. Large terrains are very poorly slow handled in WED. Here you can use GED. GED is faster.
In WED and GED you can edit Terrains.
In GED you can also paint your Terrain.

I do not recommend Imports or outstanding Editors because values often wrong (or bad to set).