We've created this Forum Thread with some workarounds and advice to assist you in publishing you ebook.
Uploading error: Differing PDF page size
I tried uploading a PDF that had a few pages of differing sizes, and I got an error message. So I redid the PDF, and the pages are all the same size. However, I still get the "we do not accept PDFs with different page sizes" error message upon trying to upload. I've scrolled through the PDF repeatedly (148 pages), and I cannot find a single page of varying size. I tried renaming the file and reuploading and got the same error message. Any advice on uploading this PDF will be most appreciated--thanks!