The evidence so far for the next iteration of the iPhone has converged on the expectation of a 4.7- or 4.8-inch iPhone 6 and a 5.5-inch iPhone, which would fall into the "phablet" category (a combination phone/tablet). The phones will likely have the same aspect ratio as the current phones (that is, have the same shape), but will have larger displays. One rumor claimed the iPhone 6 will have HD resolution of 1,920 x 1,080 pixels and that the phablet will be a 2K device, with 2,272 x 1,280 pixels. Several rumors have said that the iPhone 6 will arrive on the same schedule as usual for a new iPhone, which would mean a September launch, with the phablet having a separate launch sometime later. If you're curious what the new phones might look like and how they compare in size with the current phone, BGR has posted a concept video comparing the various sizes. It gives you a clear idea how much larger the new phones will be.
All this is still rumor, of course, but there is certainly a lot of evidence for a larger iPhone. One question regarding the phablet is when it might appear. Most rumor mongers are speculating in the fall, but I'm thinking it could be 2015. When the iPhone 4s was announced, there were many rumors of a 4-inch iPhone. But what we got was a 3.5-inch iPhone, and the 4-inch iPhone 5 didn't appear until a year later. So sometimes the rumors are quite far ahead of the reality, leaving open the possibility that the phablet could come quite a bit later.
In other news, the rumor that future iPhones will use sapphire crystal in the display instead of Gorilla Glass continues to get more credence. It's known that Apple has invested in sapphire crystal technology, and the material is already being used in the iPhone 5s Touch ID. It's extremely hard and almost impossible to scratch. Apple is building a manufacturing facility in Arizona for sapphire crystal, and 9To5Mac recently reported that Apple has purchased enough furnace capacity to make between 100 and 200 million displays per year. According to rumors, Apple is aiming at bringing the facility online this month.
Whether or not this is true, it's certainly typical of Apple to always be forging ahead with the latest technology. When Steve Jobs first decided to use the super-hard Gorilla Glass instead of what was currently available on the market, Corning said there was no way they could get their newly developed product ready in time for the launch of the original iPhone. Jobs told them to figure out a way to make it happen. And they did. Now once again Apple is taking the next step toward better quality.