A tip when resizing a terrain. To keep it at the right height after resizing, set the "z" position at exactly half of the "z" size. In other words, if the new terrain is 500 in. tall, set the vertical position at 250 in. Also, in the terrain dialogue box you have "re-scale terrain to working box" checked. If you uncheck this, then the terrain will be set to the actual size. Be careful, actual size can get quite large, really fast. Terrains are generated with algorithms, so when you set the terrain to a larger size, the scale of the features change to accomodate it.