If you are self-publishing (or vanity publishing) also sometimes known as desktop publishing, you may be keeping an eye on the bottom line.  Our company is always in search of the way to produce a great book while keeping costs down.  Because of this, when deciding on what size (finished dimensions – 6×9″, etc.) you may want to take into consideration how much information can fit on a page at what cost.  An example is that with our company a book that is 6×9″ and one that is 8½x11″ cost the same to produce, but the author can get much more text on the larger book page.  One argument is that the smaller book can have smaller margins, however when one diminishes the margins it disturbs the smoothness of the book.  White space is important to book design.  Additionally, almost every printer has some specification as to what margins he or she need and/or suggest.