The unity terrain has a light resolution for lightmaping. and that resolution you cant change. or no way I know.
How do I make it a higher resolution?
You can change the resolution for light mapping on Unity Terrains. Select your terrain object, and then open the light mapping window. In the “Objects” tab, you should see a set of options for the selected terrain. Here, you can change the resolution of the light map. Unfortunately, the highest resolution supported appears to be 4096, although that should be plenty high for small to medium sized terrains.
Hope that helps!
-Andrew
