Samsung recently introduced a pair of 12.2-inch Android tablets. The notion is pretty hilarious and has gotten about the reaction I'd thought it would, mainly because Samsung is Samsung, and most of what that they do is considered hilarious in my circles. That having been said, the idea of larger ARM devices and larger devices running "tablet" operating systems has been somewhat intriguing to me for a while. For starters, it has been a few months since the rumor mills started talking about what could be a "mega" or "pro" sized 12-13-inch iPad. The idea has been intriguing to me since the start. One of the ways Apple always tried to make the iPad usable is standing up-right, in a horizontal position, reclining at a 20-30 degree angle. (Kind of like another tablet I know of.)
In this position, the iPad is almost a good stationary computer, but the fact that audio comes out only one side of the iPad, and that the display is so small and the device is fairly unstable while sitting on a desk has made me worried about doing anything more long-term on it. I don't know if I'd have the interest in leaving it at my desk and, say, writing my entire NaNoWriMo novel on it.
However, a 13-inch display would be perfectly usable for such a task. If I had to say that I would do anything in particular with a larger iPad, it would be a device that sat very stably on a desk, either because of an included dock (more on that later) or some built in stand or another. In addition, it would have stereo speakers and some more input and output capabilities than the other iPads. I'm thinking that it may have a few USB ports, and it may even accept a wired keyboard. It could either be kind of large and fat in order to get a very long iPad-like battery life, it may have just enough battery life to save work in a power failure, or it may simply eschew a battery.
That's right: I'm envisioning a desktop iPad. Unfortunately, such a device is almost certainly going to be too complicated for Apple's tastes. The best way to implement it would be either using a stand similar to, say, a Dell UltraSharp monitor (the stand could be height-adjustable and have speakers and ports built in) or to use a docking station that the iPad sits in, and attaches to with magnets and pogo plugs.
The simpler-but-less-flexible option for a larger or "desktop iPad" would be to simply build it to favor one orientation over the other, in a manner similar to the Microsoft Surface and Dell XPS 18, which are each essentially only usable in a certain position. Even more simple than that is to simply introduce a larger version of the iPad that they've already got. This is absolutely the most likely scenario, but it's the one that is least exciting to me.
I think one of the core questions is how adventurous Apple wants to be with the platform, and how well they think it stands alone as a platform. Even on its own, a 12-inch iPad is interesting to me, because it implies a few different usage cases than the 8 and 10-inch iPads do. For example, it's interesting to consider what somebody might do with a desk-bound iPad, or whether or not such a thing would change the implied use cases.
One of the things I like to think I would ever use my iPad for is for writing. When I first bought it in early 2010 (by the way, we're coming up on the fourth anniversary of the iPad,) it cost a third of what the MacBook Air did, and the 11-inch Air didn't exist yet, and the Air got a hilarious five whole hours of battery life. Apple wasn't bundling iWork with anything at the time, but it was a $30 add-on for the iPad, and a $100 add-on for the Macintosh, and at the time, the Air lacked a few other niceties that it has today in its current position as the budget-focused "default" Macintosh, at $1000 and $1100.
Although the differential has since decreased a lot, getting into the iPad as a computing ecosystem continues to be far less expensive than getting into the Mac, at least if you want a portable system. (The mac mini and the base model iPad are similar in cost; but one comes out of the box ready to use and the other needs a display, keyboard, and mouse.)
One of the problems with the iPad Pro, as the rumor sites have dubbed it, is that it encroaches a bit further on the territory of the traditional MacBook Air. The question is whether or not the iPad stands far enough away on its own as a platform to merit a third size category. Could buyers looking to buy an 11 or 13-inch MacBook be wooed away from it by a 13-inch iPad? And, I suppose lastly, does the iPad offer enough as a standalone computing environment that getting one with a bigger display at a higher cost would be worthwhile. Even if it had more memory, bigger storage, and a faster processor; the iPad would be handily outstripped by either MacBook Air model, in terms of performance.
If you presume that the iPad can and is used as a standalone computer, I think that a 12-inch model is merited. I suppose the question is if people are going to use them on the go, or if it's going to become the equivalent of a desktop in the iOS universe. It wouldn't be the first small-screened desktop.
There are of course other questions, but I think after you've decided that the iPad Pro will be exactly like the iPad Air, but scaled up, those are all implementation details. Unfortunately, while I'm sure it'll be a good product, it's also a very boring one.