TerrainData object: How do you change a terrains size?

The question is:

On a TerrainData object, how do you change the total terrain’s size on all directions, X Y and Z? I need a bigger map and the script generates by default Too small to care about™ maps.

Select the terrain, in the Inspector will be Terrain properties. The ‘gear’ tab has Terrain Width and Terrain Height, change those.

If you’re talking about the TerrainData type accessed by script, it has a Vector3 property named “size” (eg: .size.x, .size.y, .size.z)