Most independent authors opt to upload their print-on-demand book files to Kindle Direct Publishing (formerly Createspace) or to Ingram Spark/Lightning Source. Hence, designers tend to choose book sizes that conform to the standard sizes supported by these two providers (the sizes are very similar between the two platforms. The pages that detail the standard sizes for KDP are here, and for Ingram Spark here. For a head to head comparison of the two services, check out this article. Others argue for using both services at once.