I agree with sackrat and Rosemaryr.  I use GeoControl a lot and always generate terrains at 4096X4096.  This allows me to move the camera in near or far and enlarge/stretch and still not get pixelated.  Any unseen portions, I will paint out in the editor.  If it winds up being at a distance, I'll reduce resolution.

Experiment, experiment , experiment.  You can achieve some wild results converting fractals and cloud photos to height maps.