Billy, ebooks will usually be written on a normal page size, e.g. Letter or A4. The particular display, e.g. PC or phone, will resize the PDF to fit. But whether it can be read on the phone depends on how the designer creates the PDF. Your design brief that you give to the designer needs to specify the requirement that it can be read easily on a phone as well as a computer screen. In the end, your designer will be the best person to discuss issues like page size with you.