So I changed the size of my map to 600 x 600, just to test things out for fun, I raised the terrain to create hills and mountains etc. but I munge it and the entire terrain still comes out flat. Any suggestions on why it is doing this?

Use "power of 2" dimensions. Keep it square and power of 2 and you'll do fine.

Now I am having another issue where the terrain will load, but now the textures are not showing up on the terrain. Why is this?

Edit:

Did the power of 2 size again and now the terrain isn't loading at all. Just a Yavin sky with no textured terrain

This might not be the particular issue you're having, but it's very important to note that resizing terrain after any objects, regions, paths, etc. have been placed causes a lot of problems. If that is the case for you, you're gonna need to create a whole new world and start over more-or-less from scratch. D:

In the future, if you're dissatisfied with the size of your terrain and you want to change it, AND you've already placed objects, etc., here's what I recommend you do:

1) Create a new world in Visual Munge. We'll refer to this new world as ABC, and the other world as 123. :d
2) Open up ABC in ZE, and resize the terrain to either 256x256 or 512x512 (there's very little chance you'll need anything much larger than 256x256).
3) Click the Save button or press Ctrl+S to open the Save As window.
4) Select "Terrain File (*.TER)" from the "Save as type" dropdown menu.
5) Navigate to 123's world1 folder.
6) Find and make a separate backup copy of 123.TER (just in case).
7) Rename the file you're saving to 123.TER and overwrite the file.
8) Done!