While you can find tablets with displays in a variety of sizes, the general pattern is for devices to either be 7-inches or around 10-inches. The major outlier is the Apple iPad mini at 7.9-inches, but it looks like there may be a lot more competition in that space around 8-inches because manufacturers want to avoid the surfeit of options in the 7-inch space...
Source: http://www.phonearena.com/news/You-may-see-more-8-inch-tablets-because-of-the-competition-in-the-7-inch-space_id47570
Source: http://www.phonearena.com/news/You-may-see-more-8-inch-tablets-because-of-the-competition-in-the-7-inch-space_id47570