That was an error message meant for you! :)
It (clearly) tells you that the browser doesn't allow you to make the window as narrow as you wanted. Besides that, you also get a hint on what you could do to get around that.
Basically, in this specific situation, you have two choices:
1. Zoom in (<CMD> + <PLUS_KEY>) to around 150% and try again. This works if what you really wanted was a narrow viewport (not window). Even though the window itself is larger, the viewport is measured in CSS (virtual) pixels, which would mean that you basically increase the size of a virtual pixel to 1.5 of a physical pixel.
A zoom of 200% means one CSS pixel equals 2 screen pixels, which is exactly what happens on retina screens.
2. Use the "Open as popup" tool, which is the second button in the right hand side column of the extension's menu (that button with an arrow on it), or just press "P" on your keyboard while the menu is visible.
I hope this helps!
I'm having this issue, too, and while your explanation makes sense, I don't think it's applicable. I am using version 2.0.1 Beta (188.8.131.527) on Windows with 175% desktop scaling, and I get this error for every preset, including the built-in ones, no matter what size they are. I had used the 1.x version for a long time without any issues.
I have one profile that works on my machine, and it is setting the window to 1280 x 1024. If I change the width by one pixel in either direction, it will tell me that the width is too small. The same happens for the height. Only 1280 x 1024 works. Well, 1024 x 1024 worked, too, but I have no explanation for that one. Maybe 1280 x 1280 would work? In any case, it seems like you have a bug.
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