Select a youtube resolution and resize the player.
I use this script for this: https://greasyfork.org/en/scripts/811-resize-yt-to-window-size
Sorry for the late reply. This isn't really something I can do actually. True fullscreen is something that scripts cannot activate for security reasons, and can only be activated when a user interacts with an element on the page with an event handler. While it might be possible to create a fake fullscreen that works similarly, it will likely have issues and be prone to breaking and is not something I want to try to implement.
Perhaps that user-script someone else linked will do what you want. Thank you for the suggestion though.
Could you please add an option to make videos automatically open in full-screen mode when you click on them just like how you already have an option to automatically open videos in theatre mode?