Here is what I found.
My terrain was way out of the boundaries of the cube. This changed it's size right before my eyes.
TerrainInfo Properties
TerrainInfo
TerrainScale
X was set to 64
Y was set to 64
Z was set to 64
I changed all three to 32 and the terrain was inside the cube.

I had also played with those settings... but had different results. I must have had something else sized differently because the closest I could come to matching my box was by setting those values at 50, 50, 50.

I am thinking that something else in my map was resized which effected my result.

Heres another tidbit. I made a cube 65536 square. I built the terrain and it was very small compared to the size of the cube.
I set terraininfo size to 256, XYZ. It fit the cube perfectly.
Also the terrain was very high in the cube. I moved the terraininfo icon down and the terrain moved down. I set it to where it was at the bottom with nothing sticking out past the floor.
Also I had to set the camera speed to fast. Evidently that goes back to the default (slow).

Okay guys try this. Make a fairly large cube.
Create some terrain a lot smaller than the cube.
Move it around with the terrain icon. Needs to be at floor level. I think this is the way they get terrain in levels butting up to buildings. I can't wait until I can try this later this evening.
One of the other posts I said how to change the size of the terrain. You can make it smaller or bigger by just adjusting the XYZ in the terrain info properties.