The size of the terrain created is controlled by your Gaia Defaults file, which is in the Data directory where you installed Gaia.

This file is set up to give good results on a typical desktop game, but you may want to change it for your own game, eg to support mobile.

When you check the settings out, they are identical to the settings in your terrain object, as they are passed straight through to the Unity terrain when it is created.

To change it, click on your Gaia Defaults file, and then hit Ctrl + D to duplicate it and give the new file a meaningful name. We are duplicating it so that you still have a known good configuration to refer back to.

 Gaia Defaults

You can then change the terrain size etc to suite what you are trying to achieve. So see the impact of the different settings you can hover over them with your mouse.

In general I would leave the terrain height, and detail resolution per patch as is, and would change the other settings.

When changing terrain size, I would generally try and keep the ratio's approximately between the heightmap size and the other settings the same i.e. heightmap resolution is 1/2 of terrain size etc.