Yays! is an amazing, lightweight and non-intrusive userscript that controls video playback and allows you to set the preferred player size and playback quality on YouTube. It is made and maintained by eugenox. Since Safari doesn't support userscripts, I decided to pack it as a Safari extension, so we can enjoy it as well!
Read more about Yays! on its official homepage
You can choose your preferred playback quality which is set automatically when the video loaded. Beside the standard resolutions (144p, 240p, 360p, 480p, 720p, 1080p) there are two special options: AUTO Let the player choose the proper quality. (default); ORIGINAL Set the highest available quality
You can set the default player size to: AUTO Let YouTube handle the player size. (default); WIDE Set the wide size as the default; FIT Set the player width the same as the page width, keeping the aspect ratio.
You can set the default playback state: START The video starts as soon as possible. (default); PAUSE The playback is paused but the player keep buffering; STOP The playback is stopped and the player doesn't buffer either; AUTO PAUSE Like PAUSE but only for videos opened in a background tab, and also the playback will automatically be started when the tab gets focus; AUTO STOP Like AUTO PAUSE but the playback will be stopped instead of paused;