How about a Full Screen button in the embedded YouTube videos?
 

I would use it every time I watch an embedded YouTube video. Full screen = I can lean back in my chair and still make out all the details. Windowed = I squint.

Other sites have it, so hopefully it's just one line of code.

I usually open the embedded vid in a separate tab, just click on the link above the video. Since it opens in the YouTube page, I can fullscreen it there.

But having the button here wouldn't hurt.