Some terain size and detail help plz!

I am making a fps game and I need a really large terrain. when I make the terrain larger it looses detail/voxels. how do I ad more voxels to a terrain. I tried to copy small terrain and past it and what I did in one terrain it affected the others.

please help
I have not got an awnser to the queston yet

Terrains have a maximum resolution, so if you need more, your multi-terrain method is correct. Each terrain needs its own unique terrain data asset. You can duplicate this asset yourself. A new one is also created each time you create a new terrain (as opposed to duplicating an existing one).