Video preview pane is extremely small

Inadvertently I have made some adjustment to my settings which means that when watching a video it displays only on a very thin band across the centre of the screen. The font seems to be very large too. Maybe I have adjusted the resolution accidentally? Any advice as to how I can see a larger preview of the video? I have been able to enter full screen mode by pressing the "f" key but when I exit that screen I return to the miniscule pane displaying the video. Any ideas as to how I resolve this?