For epubs, the cover is the first page of the book. If you open the book and look at the first page and there is space on either side of the actual cover image, then that is how they will be displayed anywhere a cover for the book is displayed.
If you are a calibre user, you can send the covers with the books. If you select the "Upload covers for books" option, the covers will be created on the device as the appropriate size to fill the screen. This might stretch the image. My Kobo Utilities plugin can replace the covers for books already on the device,
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