The Adjust Cover action allows for resizing and/or changing the image format of embedded cover art.
You can choose between Original, JPEG, and PNG image formats where Original just keeps the image format of the embedded cover.
Quality specifies the target quality of the image.
Max. Size is the maximum width and height in pixels of the embedded cover image. If the cover is smaller than the size provided, the size of the cover is not changed. Resizing the image keeps the aspect ratio.