To do this you would need the Heightfield Resize operator (the easiest way to add that is via the Add Operator button in the terrain node list). The Heightfield Resize has options to either resample the data or to adjust the scale that it maps to in the real world in metres.
Heightfield operators use a lot of memory though, so you might want to have a separate project for your heightfield manipulations and save the resulting .ter. Right-click on the final heightfield operator and you'll find the option to save it as a .ter file which you can read into your rendering project.
Matt
