Samsung Note 8 tablet to undercut iPad mini" (The Australian, March 19, 2013) David Frith writes that the Galaxy Note 8 tablet has a screen, about the same size as the iPad mini. David suggests Apple "hit on" the ideal size for a tablet computer. However,this is hardly a new idea. In "Australia: The Networked Nation" (1996) I predicted that tablet computers (called "PADDs" after those in StarTrek) would be about about the size of a B5 paperback book. The Apple iPad mini is 200×134.7×7.2 mm, smaller than B5 I predicted (and 2.8 mm thinner), but about the size of a trade paperback book. This is about the size of some of the wax tablets used in Ancient Greece thousands of years ago.
The size of a tablet computer is dictated by what is comfortable to hold in a human hand. So it is not surprising that tablet computers are the size of paperback books and ancient wax tablets (the human hand has not changed significantly for thousands of years). For the same reason, the size of smart phones has stabilized around that of a device with a screen about that of a credit card, as both credit cards and smart phones are "pocket size" (and pockets hand sized).
ps: The 2009 film Agora depicts students at the ancient Library of Alexandra using wax tablets in a classroom in a scene not that different to tablet computers in a modern university classroom.