Well.
I had to get an idea of the scale of the empire, and it turns out its near enough in size to Poland. That helps a lot, because although Poland isn't really the same shape as Elelion, it gives me some idea about the scale of rivers, valleys, and such like.
Here is a screen shot of the basic shape I've decided on.
Elelion basic shape.png
and here it is squashed down to isometric proportions:
Elelion map shape.png
Anyone else trying to get to grips with the relative size of things this list is quite useful:
http://www.nationmaster.com/country-...a/Square-miles