Resizing the Terrain
Open the terrain resize window by going to Edit - Resize Terrain.
Here, you can change the heightmap of the level by modifying the Heightmap Resolution setting and Meters Per Unit setting.
Meters Per Unit represents the distance between two vertices (lying on a perpendicular grid) in meters. Adjusting the Meters Per Unit will change the size of the heightmap.
Changing the Meters Per Unit to 1 will make the terrain more detailed and changing to a higher value will make the terrain less detailed allowing you to create a bigger level with the same base heightmap resolution.
The default setting of 2 means that every 2 meters there is a new point.
After selecting the desired Heightmap and Meters Per Unit, click OK, and then select the texture dimensions.