Largest Terrain Block possible with small terrain brushes
4*You will loose your terrain work, but will have a larger one to work on.
-In a nutshell, export terrain though terrain tools
-Look for your maps where it was exported too. (Yourmap_heightmap.png)
-Scale image up to 256x256, 512x512, 1024x1024, 2048x2048, 4096x4096, maybe 8192x8192 (not sure on that one)
-Import terrain, pick the heightmap you scaled and resaved but don't load the terrain preset that was saved when importing, just the one height map.
*You may get a 2nd red terrain block thats fine. Delete the one that was your working terrain, and move the new one to its new home.
-In the terrain paint tools you should see a no texture, pick one by adding it from the library and then delete the no texture one.
-Save your level.
Should be good to go.