Following a debate on Twitter between Joel, Rene, Craig, Adam, and myself, it became clear that there was still debate about the usability of a normal iPad app shrunk down to a theoretical 7.85" iPad screen.
The theory goes that the original iPad runs at 132 pixels-per-inch, and the original (non-Retina) iPhone at 163 ppi. Despite this difference in pixel density, Apple's human interface guidelines suggest a 44-pixel square  as the minium tap target size for both platforms.
This makes buttons 19% larger, horizontally and vertically, on the iPad than on the iPhone. If you subtract that padding, you end up with an iPad that is 7.85" across the diagonal, but where buttons are no harder to hit than they are on the iPhone.
That's the theory, but does it work for practical applications? Below are a few screengrabs from my iPad that have been shrunk down and then padded back out. If you load this page on a 9.7" current-day iPad, you'll find each of the below images should come out exactly 7.85" across the diagonal -- emulating how they would look on an "iPad mini". This will let you get a feel for how small those buttons get.
(Note that the resizing process has made things look a bit fuzzy. This wouldn't happen on a real iPad mini, so please just overlook that.)
(For far more thorough discussion of why 7.85", rather than any other number, and what alternative ideas Apple might use, I recommend reading Solving The 7" iPad Mini Interface on iMore and Get A Sense Of What A 7.85" iPad Mini Would Be Like on MacStories. For a discussion of why precisely 7.85" makes sense, see The Case For A 7.8" iPad by Joel. And for a detailed roundup of all the issues, as usual, John Gruber is your man.)
Technically, Apple's HIG says 44 points, not pixels. On a Retina display, 2 points equals 1 pixel. All my measurements and so forth above are for non-Retina displays, though, where the units are effectively equivalent. back