Greasy Fork is available in English.
A lightweight and non-intrusive userscript that control video playback and set the preferred player size and playback quality on YouTube.
This script uses the official YouTube Player API. That means, it works seamlessly and is probably more stable than other solutions.
Tested in Firefox 39.0, Opera 19.0 and Chrome 39.0.
Thanks to Dachaz for maintaining a Safari Extension build of this script. The package is available on his site.
You can also find this script on GitHub.
Comments and Critics are welcomed.
Sorry for my English.