I know some people who think 7" is enough, but they're not talking about tablets. Even there, though, they say a bit more is nice if the application can utilize it effectively.

I disagree that resolution, not size, matters. As adambadam wrote, it's the keyboard. I don't care if the resolution of a 7" screen is 2000x1600 or whatever. My hands are still the same size they've always been. The central typing area of my current laptop keyboard (left edge of the "Q" key to right edge of the semicolon key) is 7 1/2" wide. I wouldn't want it much, if any, smaller. That pretty much mandates an iPad-like screen for any real typing. A 7" tablet might work if you're not going to type much, but to me that means you have to carry a computer (or a separate keyboard) too. At that point, its whole justification disappears.