First of all, open Terrain / Terrain Settings (the right tab with the gear icon) and check what values you have in Terrain Width and Length fields.
If these values are very small (less than 10), scale up your model using Transform / Scale in 100-1000 times, and convert again.
Tip: if your terrains should have a size of 20x20 km, your mesh should have this size in the scene.

If the problem persists, please send me your model (as a package) by email (support@infinity-code.com).
I will check this and give you tips on how to fix this problem.

In the process, the terrain becomes faceted. I've tried adjusting resolution of heightmap and also toggling the adjust size checkbox to no avail. If I do not split the terrain I get fine results, but when I split into 20 chunks the faceting happens.

I tried to upload pics but it seems I can only upload one at a time?