Greasy Fork is available in English.
Select a youtube resolution and resize the player.
If what you are asking is to keep the video size capped to the window size even as it is resized, then yes I agree this should be available functionality. I'll see if I can find the time to do it tomorrow. Thank you for the suggestion.
I've pushed an update that will resize the video if necessary when the window resizes. You don't need to change any setting for it, this is done automatically. Let me know if this has addressed your issue.
It addresses my issue perfectly. Thanks.
Great script. Could you add an option to auto resize in theater mode on window resize?