The frame/content of output images will be different based on selected width/height and your machine screen size. Therefore, it is recommended to review the the exact content before rendering the results by switching to preview mode. What you see inside the whit border is the content. Also, if you are going to render an animation double check all frames before rendering.
You can switch to preview mode by pressing the “Tab” key. Or, by clicking the eye/pen button on the top right of the screen.