JM & David,
If you have focus on the actual YouTube player, the full screen keyboard shortcut, f, will make the YouTube player go full screen. It is a toggle so hitting it again reverts back to the player being embedded on the YouTube page.
If you have high resolution video, going full screen may be helpful. If you have low resolution video, it can make matters worse as everything becomes very pixellated/fuzzy. But you can't know what a given video will look like any more easily than by throwing it to full screen and then going back if it doesn't suit you.
Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041
The purpose of education is not to validate ignorance but to overcome it.