There are numerous tablet computers now being rushed to market for Christmas, in response to the success of the Apple iPad. Mostly these devices will be running the same Google Android operating system. But they will have a wide range of screen sizes, between the 3.6 inch screen of the Apple iPod Touch (which could now be seen as the iPad mini) and the 9.7 inch screen of the iPad. The most common sizes are 7 Inch (as 7 inch screens are readily avialable) and 9 Inch (but in a wider screen format than the iPad's 4:3 screen). But the Dell Streak has a 5 inch screen.
The market will ultimately determine which size is "best", but are there any fundamentals to this, or is it a matter of consumer taste? Some years ago I started teaching web design students at the ANU how to design for a credit card sized mobile device. I argued that this was the lagest practical size for a hand held device, as it was as large as would fit in a pocket (the size of pockets being determined by the size of the average human hand). This has now happened with smart phone, such as the Apple iPhone, converging on a size about that of a credit card.
The same approach could be applied to predict the optimal size for a tablet computer: what is a reasonable size to hold in the hand and how large are typical hand held objects? Two commonly hand held objects are bricks and paper back books. Bricks are between 203 × 102 mm and 250 × 120 mm (average 227 x 111 mm) . Paperback books between 178 x 110mm and 216 x 135 mm (average 197 x 123 mm). The width of these is determined by the human hand span needed to grip the object. This suggests a device which could have a screen of 8 to 9 inches. Given this is the size of the page of a typical paperback book, it suggests this is a workable display size.
The difference between a 7, 8 or 9 inch screen may not sound much, but the screen area increases with the square of the length and so each estra inch makes for much more viewing area. Also if a virtual (or real) keyboard is to be used, then the larger the device the better.