Small: the ideal size for your mass market paperbacks and small Kindle Fire 7 & 8 tablets. Approximate size 6.5 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 8 x 10.
Large: the ideal size for your iPad and other larger tablets. As well as hardbacks and bigger reads. Approximate size 9 x 11.
X-Large: the ideal size for your large tablets and those big honking hardbacks. Approximate size 10 x 12.
Notebook: the perfect way to carry your pretty file folders and spiral bound notebooks to meetings, classes, wherever you go. Approximate size 11 x 12.