You cannot get any extra resolution by outputting to a higher pixel dimension TER file. Ideally, especially for Terragen 2, you would output the TER (or, perhaps better, 32 bit GeoTIFF) with the original resolution, 3000x3000. The old "standard" terrain sizes for TER that GlobalMapper may still respect were only really desirable for Terragen Classic, and even that overcame the limitation eventually, allowing arbitrary-sized terrains. Terragen 2 really doesn't care of terrains are Power-of-2 size. So, again, output your terrain at the original resolution if possible, this will give you the greatest accuracy and fidelity to the original data. Scale and coverage area are handled separately with Georeference or "point spacing" data that GlobalMapper should handle properly, so even if you had a terrain of 10x10 pixels, it could still cover 100km square.
- Oshyan