Viewing Videos at Full Size
Videos are usually hosted on YouTube.com, so all the usual YouTube features apply.
Assuming a fast connection, set video quality to 1080p and use the “Pop Out” menu to view in a properly sized window for 1080p (grow the window if needed).
Separate window for video
In the web browser, right-click on the video for the Pop out choice so the video will play in its own window. It might be necessary to enlarge (resize) the window to view full 1080p quality, which is 1920 pixels wide.