Greasy Fork is available in English.
Select a youtube resolution and resize the player.
Looks like youtube made some changes that broke it. I made a change that seems to work for me for now, so please update the script and let me know if that fixes it.
Love this script. Today (also happened months before, then fixed) the autoTheater stopped working. I only changed these 2 lines:
targetRes: "large",
autoTheater: true,
Using Violentmonkey on Firefox latest.
Thanks.