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Select a youtube resolution and resize the player.
If I were to do this, I would make it a separate script rather than bloating this one with too much functionality.
I would consider making this script, but it could be pretty complex. Resizing the controls is easy, but having the progressbar functionally complete with thumbnails and all would probably require me to reimplement some of the controls from scratch. I don't have a lot of free time right now to do this unfortunately, so I can't take this up right now. Perhaps someone else has the time to make this.
I'm glad to hear that what I have done is working well for you though.
Could you add an option to resize the player buttons/progress bar? For example, making them fit the video width instead of being the entire screen width: https://i.imgur.com/nywknHk.png