YouTube Theatre Mode will take up the full height of the screen (excluding the top bar). I have stopped updating this as I have switched over to use YouTube Fullpage Theater.

It's been a while since I last made this. ytd-watch-grid and ytd-watch-flexy affects the old and new layout but I can't remember which is which. You may have to edit the 56px if your top bar is bigger for some reason. I reccomend you just use the above userscript instead unless you have a reason you can't use JS.