Though I do not use YouTube heavily, thanks to my limited internet connection ;-), one feature that I feel like missing is an option to resize videos freely. Now, YouTube does let you view in full-screen and a button to make it a bit large, still, it is handy if we could freely resize them.
If you use Firefox and looking for an Add-on that does just that, then you should try ‘Youtube.com player Resize’.
It is a newly added one and once installed (no need to restart the browser, only works with Firefox 15 and later), whenever you are watching a video, as shown in the below screnshot, it will add a small grey vertical bar around the right-edge of the player’s screen.
When you move your mouse pointer over it, then it will change into a ‘verticle-resize arrow’. Now you can resize the video by dragging it sideways. That’s it!.