If using a desktop browser, simply click the top right button (looks like a little box) and the video will expand as fullscreen.
If using a mobile device, tap the video itself and the same icon will show, also in the top right corner. Once you tap that icon, physically turn your mobile device to landscape (widescreen mode) for the fullscreen to take effect:
To minimize out of fullscreen, simply tap the symbol again (or if on a desktop, you can hit "esc"):
