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

To keep track of your subscriptions
https://chrome.google.com/webstore/detail/veefeed/elpfmfdcigaklhenpknoifgfcfnefkdf