No, there will not be a larger iPhone this year.
Update: Please read this first. There seem to be a lot of people who miss the point here. I'm not saying that Apple will never go back on their claim that a 4-inch iPhone, I'm saying that this year will not bring a larger iPhone.
Apple is already planning at least the next two iPhones. If they knew in early 2012 that the 2013 iPhone would have a 4.8-inch screen, they wouldn't have released a 4-inch screen. That would be an enormous "fuck you" to developers.
To start, I'd just like to establish that I'm not saying Apple will never make a larger iPhone. I think it's entirely possible that that will happen. I personally don't want it to, because I absolutely love the size of the iPhone 5. Hell, I loved the size of the iPhone 4S, but the move to 16:9 was worth it.
Everything that Apple does is absolutely perfect, and because of that, the iPhone will stay the same size forever. Apple says iPhone 5 is the perfect size and therefore iPhone 5 is the perfect size, right?
... except, no. And we know this because they've changed the screen size already. Apple is a company just like any other company, striving to release the perfect device just like the rest of them. They haven't done it yet, and never will. The iPhone will very likely see a change in form factor sometime in the future.
But it's not going to happen this year, and it's silly to think that it will. I'd normally feel no need to post on this, but there are tons of rumors swirling around and there are tons of posters talking about how Apple "needs to" release a larger handset, and will definitely definitely definitely do so this year.
Well, one, no, they don't need to. Apple makes more money than anyone else in the industry with their small iPhone. The only field they're losing in is the "cheap devices" category, where all of their rivals have released phones that cost a lot less off-contract. This isn't a category Apple is likely to move into, given the company's past.
And two, no, they definitely definitely definitely won't release a larger iPhone this year. And if you want to know why, here's all the evidence you'll ever need:
The entire Apple website is literally plastered in this marketing rhetoric praising the 4-inch iPhone 5 screen as God's gift to hands. The iPhone section of this website will look like this until the day of the iPhone 5S event, meaning that while the event for the new iPhone is happening, the iPhone 5 will still be advertised on Apple.com as a "dazzling display of common sense".
Changing the screen size of the iPhone one year after having claimed that it's the perfect size for a smartphone to be would not be a smart move for Apple, and when it comes to their phones, they've rarely not made smart moves.
Whether it's actually true or whether it's marketing magic, Apple manages to make every small change to the iPhone seem deliberate and thought-out. Much more so than their competition, who seem to spit out larger devices every year not as a deliberate choice, but as a side-effect. "Can't fit LTE radios into our phones? Let's do 4.3 inches! Can't fit 720p screens into our phones? Let's do 4.7 inches! 1080p, you say? Aw, hell, let's do 5 inches!"
Changing the screen size after just one generation would be Apple saying that they made a mistake by going with 4 inches. Apple would not let that happen to their reputation.
Apple never said that the iPhone's 3.5-inch screen was perfect for one-handed use. The fandom said that. This 4-inch screen marks the first time that Apple has ever come out and claimed that a screen size is the ideal. There's absolutely no chance that they'll go back on that later this year.