Mockup of iPhone with 4.94-inch screen, created by Marco Arment.
Apple's apparent behind-the-scenes tinkering was revealed on Monday by The Wall Street Journal, which indicated that such devices have been tested with Asian suppliers. While a specific screen size for a new iPhone model was not given, it was reported that the company has specifically built a prototype iPad with a screen sized "slightly less than 13 inches diagonally."
There have been reports in recent months that Apple could be planning to build an iPhone with a bigger display, measuring around 5 inches diagonally, as soon as next year. It's believed that Apple is exploring such a device in response to the success of jumbo-sized smartphones like Samsung's Galaxy Note series.
Apple already increased the size of the iPhone display from 3.5 inches to 4 inches with the launch of the iPhone 5 in 2012. The company boasted that by making the new screen taller but not wider, it can still be used with one hand.
But even at 4 inches, Apple's iPhone 5 remains one of the smaller smartphone displays on the market. For example, Samsung's new flagship Galaxy S4 features a 4.8-inch AMOLED screen.
While rumors of a so-called "iPhone Plus" are not new, word of a larger iPad has been less common. While smartphone sizes have trended larger, tablets have skewed smaller, as seen by the success Apple has had with its 7.9-inch iPad mini.
Apple is expected to launch a redesigned full-size iPad later this year, but it's believed that device will sport the same size 9.7-inch screen as every iPad that has come before it. A 13-inch iPad would put it in a similar class with the MacBook Pro and MacBook Air, both of which are available with 13.3-inch screens.
Noted insider John Gruber of Daring Fireball said in May that he expects Apple to release an iOS-based notebook "eventually" ??speculation that may align with Apple's alleged interest in a 13-inch touchscreen panel. Gruber said he does not, however, expect Apple to ever release a touchscreen MacBook.
Accessories that turn Apple's current iPad models into laptop-like clamshells with keyboards have proven popular among accessory makers. Microsoft has also touted the laptop-like functionality of its iPad-competing Surface tablets in television advertisements.