Sorry, misunderstood your post.
I think it should normally be in the list if supported by your graphics card and/or your screen.
But why do you want to change?
If you want to go full screen: depress [F] (= toggle switch)
If you want to pan-in or out: depress Num+ resp. Num -
Or press [O] (letter-key) for auto pan scan (or [Shift]/[O] for custom pan scan).
(Num-5 to reset)