The reason you get a bunch of things telling you it doesn't know the size is because Derelict only users a forward declaration for sfWindow and sfRenderWindow. It never actually defines it, so D doesn't know how much memory to reserve for the object. This is ok because you don't actually need to know.
If you instead use a pointer, you will be fine:
sfRenderWindow* window
All of the C functions with windows and render windows as parameters take a pointer to the window, not a reference to the object. Likewise, the functions that create windows return a pointer to a window object.