I did way more then 20 of these high res terrain skins for Starhawk (lost count ... and there were a lot of terrain files for shattered planet environment)
Core terrain was 4906 ft x 4096 ft (16777216ft² = 0.60179982mi²)
That doesn't include the 'infinite skirt'
All source was generated at a whopping 8192 x 8192 resolution
16-bit heightmap based terrain...
... but we baked (with tools and pipeline I designed) to low-res mesh
... generate displacement maps
... then in realtime sub-D the terrain from the low back to the target resolution based on distance
Maya side tools for wrangling data, building shaders, etc. (automated)
Tools also generate all the deco placement on terrain (based on map inputs and deco project configs)
Tools output deco placement maps, so I can bake some deco occlusion into the texture inputs
Terrain uses blend maps and detail maps for high-res fidelity at player scale
We used a novel in-house solution for 3D projection/blending of multiple detail layers so we could have awesome vertical canyon walls, etc!!!