Ideally, the perfect size for your featured image is 1920 pixels wide by 1200 tall, although any image that's at least 1200 pixels wide can be uploaded into the editor. The editor won't allow an upload of an image larger than 5 MB.
If you're having trouble finding an image you want to use that meets these requirements, try using the Unsplash integration by clicking the "Unsplash" option in the featured media section. This will let you search Unsplash's library of royalty free stock photos, all of which will work perfectly in your Vocal stories.