how to delete the unwanted terrain space while designing a level?

Hi

We are working on a 3D side scroll on Unity3D.We are developing the levels for the game (3D side scroll).All i need is to delete the unwanted terrain space for my side scroll since the side scroll is using only 25% of the terrain placed in the levels.

I would like to know the procedure to delete this unwanted terrain in unity3D so that i can win over in using a minimum hard disc space for my game.

In another case i want to copy the texture of one terrain to the adjacent which is the copy of the previous one.I am not able to copy the terrain texture on to the other terrain WHY?

Thank you

Siddhartha