I’m often advising clients that they should make sure that they have a business page on Facebook. If they have information that they want people to know about, it’s another handy avenue to generate interest and business.
I just stumbled upon a page that helps you to know what size the images that you use for the profile and cover sections should be. And now that you want to be aware of smartphones too, they also show where images will be cut off on smaller devices too, so you can plan accordingly. Check out https://www.facebook.com/PagesSizesDimensions