You will need to install a user script manager extension to install this script.
New: Support Picture-in-Picture(Chrome 70+).Default hotkey F2.
This script allows to expand the video to the whole page size.
It supports videos from almost all pages. Also supports complex multi-page iframe nested videos, HTML5, flash games etc.
You can revert the video back to normal clicking on the far right or left of the screen or by pressing ESC.
If the maximize button does not display or any other problem, try to press ESC.