The release of the 7.8 inch Kobo Aura One raises the question on what makes the right size to read PDF files. The answer depends on use-case scenarios. For example, many humanities books come in an A5 size and the 8 inch form factor is ideal for these books. On the other hand, science books and complicated PDF files, with a two column layout, require closer to 10 inches to be comfortably read. Similarly complex magazine pages are better suited to larger colour tablets.
Personally, I find the eight to nine inch e-reader to be the right size. At this size there is the right compromise between size for comfortable reading and portability. It is not too small, e.g. six to seven inches, but not too big for one-handed reading. The portability and convenience of a smaller form factor makes a big difference for long reading sessions, especially outdoors or in tight spaces on public transport. I prefer to read in portrait but can switch to landscape, if half a book page is viewable and text size is legible. With six inch e-readers the landscape view often gives you snippets or small text and it is easier to lose your place within a larger book.
Another important factor to consider, especially with the eight inch e-reader, is the right software to optimise reading with an eight inch screen. With KOReader, for example, it is possible to set reading to scroll mode and alter PDF page margins. Accordingly, many PDF files can be comfortably viewed and navigated, considering the restrictions of a smaller eight inch screen, in portrait mode.