You would want to have the settings set to 16:9 ratio. If the image still looks off you should be able to change the settings on the external device to rectify this. Most cable boxes have this setting within their settings menus to change this.
I have a samsung 40 inch led tv. with the 16/9 setting my picture is to large for the screen and get cut off on all sides. how do i fine tune the picture size?