Click on your project folder (Picture below)
and change the value of the window size (Picture below)
And put in there the resolution you want, just like 'Lancifer' wrote above me.
[quote:1mejc15j]18:9 is the same as a 2:1 ratio.
Just make the Window width twice the size of the Window height and you will have a 18:9.
(example: 720x360, 960x480, 1440x720, etc.)