Breve terrain

I've built a custom terrain generator trough plugin use but the results I get in breve are far different from the real terrain generated.

From what I read from the documentation the set-height works with polygons and not real world locations. How does this work? I ask because my generated terrain consists on a (double) matrix of height points and setting the height of the polygon to those points might be whats wrong.

Also how do we limit the terrain size? Apparently having a size of 16 or 256 yields the same results (world of constant size).