Thank you!

Terrain resolution also matters, more pixels on heightmap = more triangles on terrain

The maximum dimensions for a Unity terrain tile (if I remember correctly) is 100,000x100,000x50,000 meters.
To make something 500x500 miles you'll need several terrain tiles.

I don't know exactly how the game would handle that much terrain. SimpleLand uses procedural generation and runs pretty well on my laptop, but I don't know how well standard devices would handle standard terrain of that size.