This style forces to use the pre-flexy video size before it first came out in 2018.
Both of these are required to install here (you need Tampermonkey before you do this)!
YouTube Video Resize Fix Script (It also fixes the video streaming and controls cut off issue)
https://greasyfork.org/en/scripts/457319-youtube-video-resize-fix
Custom Aspect Ratio Userstyle (Make sure you can set it to 16:9)
https://github.com/sapondanaisriwan/AdashimaaTube/blob/master/userstyles/features/custom-aspect-ratio.user.css
And also, you can edit both min-width and max-width sizes on the ytd-watch-flexy part depending on your display resolution like '426', '640', '853 (Default)' and '1280'.
