The YouTube Feather beta allows you watch videos without a lot of useless features (no comments, Google+ crap, ...) The only downside is that they only offer the small video player. A simple bookmarklet solves this easily.
The bookmarklet has a customizable width for the player. For this example, I chose a width of 900 pixels. We'll set a new size for the container and video player, along with the correct height (16:9) for the video player. An extra height of 30 pixels is added for the player controls at the bottom. Finally the left margin is reset to 10 pixels.
If you want to adjust the width, change the w variable at the beginning.
document.getElementById('p').style.height=(w*9/16 + 30)+"px";
Simply drag this bookmarklet: YT900 containing the above code in your shortcut bar
Here's the bookmarklet for a 1280x720 resolution: YT720p