what I did was insert an image in the "key" for the current page "cover text". This way the size of the cover is dictated by the size of the image, nothing else. Also you can make the image resize with the size of the page with a bit off css. So if you have different images you want to place in the cover for individual pages the "key" text function works well, not sure if this is recommended by the devs, its just how I did it.