Once the terrain has been created its size can not be changed.
To change the size of new terrains go into your GaiaDefaults file in the Gaia / Data directory and scroll down to the resolution settings section.
Change the resolution settings there - hover over the settings to get a description of what they do.
My general rule of thumb is to set the terrain to the desired size, and then make all of the other settings half of this.
- Don't touch detail resolution per patch unless you know what you are doing. Valid settings are generally 8 or 16.
- If you are looking to target mobile devices or VR then consider dropping the resolutions down to settings in the range of 128 or 256.
- The higher the resolutions are the more accurate the terrain is, and the slower it performs.