Gaea supports setting an arbitrary scale for your terrains through the new Terrain Definition panel. If you're a casual terrain artist, this won't affect your workflow too much — the defaults will be sufficient. But if you're creating precisely scaled terrains, the Terrain Definition is your best friend.
The Terrain Definition popup is found in the main toolbar.
You can define the Base and Height of your terrain in meters. Both are counted from sea level (0.0m). The Scale determines the extent of the entire terrain. By default, Gaea terrains are set at sea level (Base: 0m), 2.6km tall (Height: 2,600m), and 5 x 5 square kilometers wide (Scale: 5,000m).
// Internally, the terrain scale calculation is performed like this: (height / scale) - base
A height of 2.6km is usually more than enough for most terrains. For perspective, Mt. Everest is 8.8km tall! Going that tall is not recommended unless you really know how to handle the side effects of such tall terrains.
Terrain Definition is non-destructive. You can scale your terrain and the update will be applied on the fly without requiring a rebuild of your terrain. You can change the values as often as you like.