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.
One of the benefits of digital printing combined with online bookselling is the capacity of printing books as they are ordered, rather than pre-printing and warehousing. This massively reduces costs for small presses and self-publishers. Two major players dominate this field — Createspace, owned by Amazon, and Ingrams Spark (Lightning Source in Australia). The two services offer a very similar level of functionality, but there are differences in pricing, approach, assistance and sales channel availability. Check out these interesting articles discussing the relative merits of each service, and identify which one better fits your particular use-case.