Depends, size can be huge if you do things proper.
As far as steps go.
Learn a GIS program. Make the terrain using that, or world machine, or anything that manipulates height-maps.
Assemble the height-maps in Houdini (or any DCC you prefer). Slice them up and import into the engine for assembly.
Rough sizing should be 1km^2 parts, well below a total vert count of 1vert per meter.
Then you need to create a replacement HLOD system that will swap out distance pieces with aggregated pieces (which are also reduced in vertex count with proper lod).
Doing it that way, you can get even the Switch to do massive things…