If you watch a lot of videos, you may find these useful.
Use this one for youtube: https://greasyfork.org/en/scripts/811-resize-yt-to-window-size
If you want to control the youtube and twitch players with keyboard shortcuts, I also have these https://github.com/fent/dotfiles/tree/master/Scripts
To move videos to another "player" window as they are opened, and pause any playing videos, https://github.com/fent/chrome-vsnap