Go into your settings. Under 'live chess' there is a button that should read something along the lines of 'always use focus mode'. It's not totally full screen but it's a bigger board and fewer distractions on the screen.
Full Screen

If you play on a Windows computer, you can put the browser window in full screen mode by pressing the F11 key.
How to Get an Internet Browser Out of and into Fullscreen (computerhope.com)
Can someone please teach me how to play with a Full Screen, i.e. with nothing but my game showing on the screen?
My screen name is "breadalone", or text me at 203-247-2615, or email breadalone@msn.com.
Thank you!