1400 pixels; I originally saw it as a warning for iTunes Producer. Article link here (and I'll update the post too): https://www.google.com/search?q=“http%3A%2F%2Fwww.teleread.com%2Fpaul-biba%2Fnew-ebook-cover-image-requirements-coming-to-satisfy-higher-resolution-e-reading-devices%2F”&btnI