Once upon a time, Apple (AAPL 0.55%) boldly proclaimed that a 9.7-inch display was the perfect size for a tablet. The Mac maker proved itself wrong when it launched the 7.9-inch iPad Mini to much fanfare, and the smaller variant's unit sales have risen to overtake the flagship version.

In the spirit of further growing its product families to address a wider range of market segments, The Wall Street Journal has it on record that Apple is testing out even larger displays for both the iPhone and iPad. The idea of a larger iPhone is nothing new to investors, as rival OEMs have enjoyed demonstrable success in the growing phablet trend. There's no rush for Apple to launch a larger iPhone, since phablets are still very much a niche (but growing) segment of the global smartphone market.

However, this is perhaps the first speculation that Apple is thinking about going even bigger with the iPad. The Journal says that Apple has prototyped iPad displays close to 13 inches, which would put the tablet closer to the size of a MacBook. Apple tests prototypes all the time that never make it to market, so a larger iPad still might never become a reality.

While consumers are clearly shifting toward smaller tablets, it remains a distinct possibility that Microsoft's (MSFT 0.34%) vision of computing will find a place in the future. The software giant has been espousing the idea of "PC-Plus," where tablet and PC form factors converge over time.

This has spawned a movement among Windows OEMs of every imaginable form factor, including hybrids and convertibles alongside traditional tablets and PCs. It's true that neither Windows 8 nor the Surface RT has impressed consumers overall, but Microsoft's fundamental vision may still be relevant; Microsoft just needs to improve its messaging and execution.

A larger iPad would inevitably be positioned more as a PC replacement than current models, albeit one likely running only iOS. If the rumored device is ever launched (a big if), Apple would be acknowledging that Microsoft's vision has some value -- except Apple believes it can execute it better.