here a useful Macro to easily set up the aspect ratio of your footage
just type the ratio you want in the Ratio control, you could do it like
2,35or typing for example
Then you have to decide if you want the output to be Letterbox or Cropped. If the ratio you input exceed the height of you video (for example if you have a 16/9 video and you output in a 4/3) you should select CROP otherwise you won't see any change.