Small: the ideal size for your mass market paperbacks and the small Kindle Fire 7 & 8 tablets. Approximate size 6 x 8.
Medium: the ideal size for your Kindle, Kobo, nook, iPad mini, and other tablets. As well as indie paperbacks, and smaller paperbacks. Approximate size 7 x 9.5.
Large: the ideal size for your iPad and other larger tablets. As well as hardbacks and bigger reads. Approximate size 8 x 10.
X-Large: the ideal size for your large tablets and those big hardbacks. Approximate size 9 x 11.