Thanks for being with Samsung and for sharing your concern. We would like to inform you that this fit to screen option is removed from the application and a new feature has added "pinch to zoom", under this the user while playing videos can pinch the screen via two fingers to adjust the resolution of the video which further depends upon the actual quality of the video.
For more information kinldy coordinate with the youTube support. Be sure to hit ♡ when you find an answer that works for you.
