Attached Link: http://www.renderosity.com/mod/gallery/index.php?image_id=1806409

Silverblade is correct. You change the size of the terrain like you change the size of any other object.

Regarding procedural terrains, do not increase its resolution. Like Silverblade says, it has no effect on the image itself ... but... and it's a BIG but... it will use much more of your memory in the openGL preview, without any real benefit. You could easily run out of memory.

Quote - "the resolution only affects it when you paint shapes on it with a brush, I believe."

I have some doubts about this. In this image I posted a few days ago (link above), the cave was made painting on the terrain with a brush. That's a procedural terrain, with the standard resolution (256x256) and you see no polygons at all in the cave borders (although that's a small cave in a big terrain). So, I think that the resolution is meaningless even for this case.