Resizing the Output Window?

[Ianr]

Is there any way of resizing the output window.

I am using an LED Screen 9m x 3m (3:1) ratio and would like to feed it the correct picture so nothing is stretched / Squashed at the screen. As far as I cansee the output window is always the same size?

Thanks

[Ianr]

Sorted I was looking in the wrong place for the controls