my terrain isnt getting bigger please help

im new to this so i dont know how to use this very well ive checked on youtube,questions on this website, and just about everywhere else all i wont to know is how do i get my terrain bigger

Have a good read of this- There is a section there about changing various setting on the terrain, including one for changing the size. One problem is, if you do this after modifying the terrain, you will lose your heightmap (because it gets overwritten by the new, different-sized one).