Since you are doing a top down, just go with the standard farthest distance (1e+006). It's possible that your farthest distance of 100m is not far enough to reach the ground. Next, I use a much smaller width/height, like 300-ish width. Also, keep the detail medium, like .5. And make sure that you render from the Render Sequence tab, but I'm sure you knew that already. Can't help with Blender import though. Tiny files are okay if you get enough detail/triangles in your terrain.