@leahmaps -- Unfortunately, there is no such ratio for us to provide because stories don't scale, they are responsive. That means the content layout changes depending on the viewer's screen size and aspect ratio.
You can observe this if you open the story builder and with any of the cover options enabled, drag the bottom corner of your browser window around to resize it to various shapes. You'll see that the media panel on each cover style changes aspect ratio to fit the available space.
The best options for cover images are photographs that can be cropped around the edges without losing any critical piece of the image. You can also set a focal point on the most important location in the photo to ensure that it is always visible.
Take a look at our gallery for lots of examples of great cover images: ArcGIS StoryMaps Gallery | Examples from the StoryMaps community
If you have a particular image that you are trying to use and can't, could you share that with us so we can better understand what look you are going for?
Owen Evans
Lead Product Engineer | StoryMaps