Okay, first lets talk about a large scene setting. This is where you will be able to create a terrain that is 2,000 ft tall. Default terrains in the smaller size scenes are a lot smaller. If you want to work in metric, that's fine, same principles apply. Open a large scale scene,  insert a default terrain. In the terrain room the height will say 1,500 ft, but in the assembly room it will say 1,870 ft. The reason is because of the effect of the shape filter and whichever generate filter you use. The size in the assembly room is the exact height, the size in the terrain editor is based on the generate filter and doesn't take into account the shape filters. You should start with a generate filter to get the basic type of terrain you want. This is where you can set a basic height. It won't be exact, because feature size and other filters will affect the height. If you want to create a tall  and really wide terrain, you can increase the width and breadth in the world size settings.  Any settings you apply in the terrain editor will be recognized by the shader. If you resize the terrain in the assembly room, whether it be by numeric input or dragging the scale tool, those changes will not be recognized by the shader.

Short version:
Start with a large scale scene, insert terrain, use generate filter to set a basic height.

Use other filters to get the look of the terrain you want, check height in the assembly room and adjust generate height if you want get closer to a specific size.

Set up the shader to look how you want it to look, then adjust the exact size in the assembly room to make it a specific height.

Working with heightfields in the terrain editor is not an exact science because you are mixing randomly generated fractal filters and other functions to create the surface. Carrara has a fairly decent amount of flexibility for random terrains, but not so much control over specific placement of features. If you really want to get deeper into terrain generation, I suggest you look at GeoControl. Right now Johannes is recovering from an accident, so I don't think the public beta of GeoControl 2 is available for a little while, but you can look at the site and buy GeoControl 1 I believe.