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. 


NOTES:

  • 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.