We've created this Forum Thread with some workarounds and advice to assist you in publishing you ebook.
Large Hardcover format for retail distribution
Looking through my choices, I get the impression that the only hardcover formats allowed for external distribution are 6 x 9. Why are the larger formats (8.25 x 10.75, etc) not allowed for external distribution, although they can be printed? I need to stick a ISBN to a large hardcover book.
Large hardcovers are definitely available for sale, in general. Most textbooks are large page hardcovers.