I have a terrain resolution 20000 by 20000 is it okay to use this one??Also at start it was 2000 by 2000 and i made some mountains in there but when i expanded it they went blurry and now i cannot change there size to match the terrain size ??Am in deep trouble help with this terrain thing??:((
The resolution for terrains has to be a power of two plus one, and it won't currently import beyond 2049x2049. If you need higher resolution, you have to use multiple terrains.
I did not understand the question well but I think your problem is you made a terrain, changed the size the mountains are too big and the mountains are blurry.
You could always make your terrain smaller and the mountains will go back to normal size. As for the blurry mountains. Its just in the editor. If you acctually play the game the mountains will look normal. I had a lot of trouble with this in unity 2.6 If you want a bigger terrain. Just add other terrains beside it(With the size you think you will need like 20000 by 20000).