The default full screen button in the YouTube video player expands the video such that it covers your entire desktop area and all existing windows on the screen stay hidden as long as you are watching the video is this "maximized" state.
That’s one way of doing things but what if you want more control over the size of the YouTube video player, here’s a simple hack – just replace the word "watch" in the URL of any YouTube video with "watch_popup" and the video will expand to fit your browser window, not the desktop.
For instance, if the original YouTube video is located at ..
the same video can be accessed standalone outside the YouTube website at..
Resizing the browser window will also resize the YouTube player and you can press F11 in Firefox to mimic YouTube’s full screen mode even with this hack.
YouTube & Sticky Windows
You may also use the trick for watching YouTube videos in sticky mode. Just open the YouTube video in a new window (not tab), resize it according to your need and use a program like Always on Top to watch videos while working on other tasks.
Embed YouTube Player
You can use the the watch_popup URL with the IFRAME tag to directly embed any YouTube video on your website without having to calculate the confusing height and width ratios necessary for the EMBED tag. In the case of IFRAME, specify the height or width as 100% and it will auto-expand to fit the entire available space on the site.
Others may use this hack to watch adult YouTube videos without logging.