Once you click to play a video, to switch to full screen mode:

1.  Hover your mouse over the video player.
2. Click the full screen icon located at the bottom right of the video window (two opposite pointing arrows in a magenta circle). 

Exiting full screen mode

To exit full screen mode, click the small screen icon (two arrows pointing at each other in a magenta circle).

Alternatively, you can exit full screen by pressing the Esc key.