OK, I managed to duplicate your setup. Unless I've misunderstood you, you're already in full screen mode and you want to resize the window right?
All you have to do is to click the 5th button along from the left (the one on its own) as shown in your screenshot. If you hover the mouse over that button it will say "Toggle the video out fullscreen".
Indeed I was already in full screen mode, sorry if I forgot to mention it (but who doesn't watch in fullscreen mode ?
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I tried the mentioned button but it removes the full screen mode, if I click on it again the issue remains.
Thank you a lot for your feedback