You may see more 8-inch tablets because of the competition in the 7-inch space
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.
The iPad mini is the big seller in the 8-inch space right now, although Samsung has the Galaxy Note 8.0 and the recently revealed LG G Pad 8.3 are also in that segment. There are also a number of 8-inch tablets on the way with the Windows 8.1 update that is opening up the smaller tablet space for that platform. The smaller Microsoft Surface planned for this year is said to be 8-inches, then there is the Dell Venue tablet, and a Toshiba 8-inch Windows tablet.
Of course, manufacturers will have to be careful with how they price these 8-inch tablets. 7-inch tablets have been popular because Google and Amazon have been selling devices at very low margins, meaning prices around $200. 8-inch tablets have ranged from around $300 all the way up to $400 with the Galaxy Note 8.0. In general, 8-inch tablets has not been viewed as a "premium" space, so manufacturers are likely going to aim more for the $300 range.