You can only import maps as grayscale images. 1080 * 1080 px as .png, .tiff and a few others, as I recall.
For each of the 256 grayscale colors (8-bit), you raise the height by 4 meters. So in total, 1024 meters elevation is the limit. You set the water level (if any) in the map editor, not in your image!
A map is 9 * 9 tiles and by standard, you can only play 3 * 3 tiles in the middle of the map. Each tile is 2 * 2 km, so you only have 60 * 60 px to define each 1 x 1 km square. Quite limited, indeed.
You *can* import grayscale images, larger than 1080 * 1080, but they will be resampled by the editor, on import.
You *can* use 16-bit grayscale .tiff, but you do not gain much higher quality, in terms of slope accuracy. Again, because you have so few pixels available.
If you want to use CAD of some sort, use 3ds Max, where you can bake a grayscale image, from a mesh.
Please anyone - correct my numbers, if I remember wrong, it's been a while since I made a custom map.