title_body: What page size maximizes readability across common ebook readers?
upvoted_answer: This would be extremely difficult since the broad range of devices out there have vastly different physical screen sizes and aspect ratios. Your best bet would be to use a small page size (perhaps 4" x 3") so that users with e-book apps on their smartphones would still be able to comfortably read the text. This comes at a price: e-book readers or tables with large screens will display very large text - although some might allow you to zoom out or display multiple pages at a time.
  
  I would suggest a slightly different route based on this similar question. Instead of exporting from LaTeX to PDF, export to HTML. Then use a tool such as Calibre to convert the HTML files produced to a MOBI or EPUB file.