Thank you for your suggestion.
Since it is not supported natively, looking at the plugins was in fact the solution. I think that the suggested plugin does even more than what I need because it adds also some new background to the cover. To just resize without changing the appearance of the cover I've found another plugin from the same developer which is called Resize Cover.
So, if it can be useful for future readers:
Ctrl+A, click on the Resize cover plugin and in few seconds all covers will get resized at the same width and height. Problem solved!
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