6 inches is a good compromise size, actually. Remember that, unlike a paper book, fonts are enlargeable in an ebook, so the issue of making pages big enough to host letters big enough to be readable is not an issue here. In fact, there are people who are very comfortable with screen sizes of 3.5 or 4 inches (smartphone users) and there is a good market for really pocketable readers of 5 inches.
However, I adhere to K-THom's opinion. Books made for advanced reading require much larger sizes. Eventually, I think they'll require double-screen readers in order to be fully functional.