It opens empty. The blank-page icon at the top left makes a new map, and the New Map dialog is where the size and the tileset get chosen.
The longer side has to be 64, 96, 128, 192 or 256. Anything else and the minimap comes out the wrong proportion in game, disagreeing with what you are looking at. 128x96 and 192x144 are fine; 50x100 and 128x144 are not. It is a nuisance to change once the terrain is down, so get it right first.
| Badlands | The default. |
|---|---|
| Jungle World | Jungle - the most common in use-maps. |
| Space Platform | Space. |
| Installation | The indoor facility tileset. |
| Ash World / Desert / Ice / Twilight | Volcanic, desert, snow and twilight. |
The Layer control on the left decides what you are editing - terrain, units, locations and doodads are separate layers. If clicking selects nothing, the layer is almost always the reason.
There is a bug where the minimap does not draw. Resizing the left pane once brings it back.