I'm pasting part of my reply from over on L3DT here in case it helps anyone else:
if read the area where you were entering size for the terrain you will note that it is under "New Terrain" which is *not checked*. That means those values have no effect in this case. You can test this by changing the values, keeping your camera static looking at the edge of the terrain. You will note that nothing happens.
So the point is that all your adjustments need to happen in the texture. Why change the size of both terrain and texture at the same time? That's like trying to paint a house, finding out you don't have enough paint, going to buy more but also switching houses at the same time.
Anyway there are two things that you may still be missing. First in the Image Map shader the size is specified *in meters*. You want to match that to the size of your terrain *as it already exists straight out of L3DT* in meters. So if your point spacing in L3DT is 10 and you have a 1024 sized terrain, it's going to be 10024 meters to a side. *It does not matter that your texture is larger in pixels*, you are specifying size in meters and as long as you specify the same size for the texture it should work.
The second thing is that you want to make sure the position of the texture and heightfield is the same, and is referenced from the same point. If you go to the Heightfield Shader which the Heightfield Load connets to you will see it's at 0,0 by default and *Position lower left* is checked. The same is true of the defaults for the Image Map shader, so it should work right off the bat as long as you specify the size correctly, but just in case you've changed it for some reason, make sure it's "Position lower left" and 0,0 for position.
Finally, if you're having trouble determining the actual size in meters of your terrain, just load it into TG 0.9 and go to Size on the Terrain window. That will tell you.
I hope that helps.
- Oshyan