While it might seem like overkill to dissect reports about the size of a still-unreleased-and-unannounced phone’s screen, when it comes to the iPhone, the screen is kind of a big deal, especially to all the developers behind iOS apps. The screen size determines the most important things: The layout, appearance and user interaction elements of an app. As a handful of high-profile iOS app makers told me recently, they have faith in Apple not doing something crazy or drastic that would upset the established development process.

9to5Mac’s report on Tuesday seems to give added foundation to that faith. Though the report concerns a test device and not a finished product, the scenario laid out is one that would be most amenable to developers and iPhone buyers. Here’s what they learned about the prototype:

Both of these phones sport a new, larger display that is 3.95 inches diagonally.  Apple will not just increase the size of the display and leave the current resolution, but will actually be adding pixels to the display. The new iPhone display resolution will be 640 x 1136.  That’s an extra 176 pixels longer of a display.  The screen will be the same 1.94 inches wide, but will grow to 3.45 inches tall. This new resolution is very close to a 16:9 screen ratio, so this means that 16:9 videos can play full screen at their native aspect ratio.

We’ve also heard that Apple will be taking full advantage of their new pixels. Apple is currently testing builds of iOS 6 that are custom-built to the new iPhone’s display. These builds include a tweaked home screen with a fifth row of icons (besides the stationary app dock) and extended application user interfaces that offer views of more content. Apple is able to pull this off with the same sharpness as the current iPhone Retina Display because of the additional pixels.

Though it’s likely we won’t see a new iPhone until the fall, Apple’s Worldwide Developers Conference takes place in June. It’ll be interesting to see if Apple chooses to preview the next version of its mobile software, iOS 6, at that event, and if by doing that they’ll end up spilling the beans on the screen-size changes months before the new device is ready.

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Analyzing the wearable computing market
• Takeaways from mobile’s second quarter
• Confused about the wireless markets? Here’s a breakdown

The Wall Street Journal and Bloomberg News reported separately last week that Apple will introduce a screen of “at least 4 inches diagonal” or “bigger than 3.5 inches,” the current screen size. What’s not clear is how Apple would go about this and satisfy both users — who may enjoy a larger screen but also want to keep the Retina display quality — and its army of third-party developers — who don’t want to have to develop for a wide variety of different screen sizes.

There are several theories out there as to how Apple could build a larger-screen iPhone. They include scaling up the screen and just dropping the pixel density a bit; changing the aspect ratio, increasing the screen height, and keeping the pixel density the same; increasing the width and the height; or going bigger than 4 inches and using a standard 16:9 resolution. Rene Ritchie at iMore has a good explanation of how Apple could get there.

But what do the developers who will have to deal with the implications of a larger screen think? I talked to a few iOS app makers, and interestingly, many of them are not convinced Apple would actually “pull an Android,” as one developer put it, and make it more complicated to design for a bunch of different screen sizes (iPhone 3GS, iPhone 4/4S and something new).

Sam Shank, the CEO of Hotel Tonight, a hotel-finding app for iOS and Android, said if Apple does change the screen size, he believes the phone itself would become slightly wider but not much taller. But he doesn’t believe Apple would mess with either the aspect ratio or pixel density.

“Changing the aspect ratio would be a lot of work for development teams. We found that maybe 50 percent of iOS development is [spent] in layout,” he said in an interview on Friday. “If we had to do two versions of that, the current aspect ratio and a new aspect ratio, and had to lay it out again, that would add considerably to our development time.”

Lenny Rachitsky, the CEO of Localmind, a social discovery app for both iOS and Android, said he doesn’t see Apple creating fragmentation problems for its developers. “We haven’t put a lot of thought into it yet, partly because we assume Apple isn’t going to pull an Android and fragment the device market unnecessarily. If they do, they must have a really good reason to do it, and we’re confident they’ll provide tools to make it easy to migrate. The last thing they want to do is put more friction in front of developers.”

Localmind’s lead iOS developer, Nelson Gauthier, sounded equally confident that Apple would make any transition, if needed, smooth. “Apple often changes the requirements for iOS applications but they usually give developers fair warning and decent tools to accommodate new devices. The transition to Retina display and iPad were both relatively straightforward,” he said.

Gauthier said in an e-mail:

A bigger display could mean a few things. It could mean that the display has a higher resolution while retaining the same aspect ratio. The new display might be a different shape. Finally, the display might simply grow while retaining the old resolution. But changing the resolution or aspect of the display need not have an effect on user interface elements, just their layout. Bars and controls could stay the same size while content areas expand to fill the additional screen real estate.

Apple might help developers transition between form factors by providing a system that would automatically adjust layouts for new resolutions and display aspects. Last year at WWDC, they revealed a component for Mac OS X called Cocoa Auto-Layout that allows developers to define layouts using fairly simple, freeform constraints. A system like this could also work on iOS.

Massive Damage Inc., which makes location-based iOS games, is also betting against major changes for developers. As CEO Ken Seto points out, there are already two resolutions many developers have to worry about for iPhone games: standard resolution for older iPhones and Retina for the iPhone 4 and iPhone 4S.

“I can’t quite imagine how they will introduce yet another retina resolution standard. Overall, I think if Apple does increase the screen size, they will be careful to not introduce too much fragmentation. They will likely keep the aspect ratio the same so developers won’t have [to] refactor their UI,” he said. “I have a feeling a bigger iPhone will automatically scale up existing retina resolutions as the screen would likely only get a little bit bigger.”

But the feeling that any changes would be negative isn’t necessarily universal. Donnie Dinch, the co-founder of iOS last-minute ticketing app WillCall, said because of the content of his company’s app, a screen size change may not have a huge impact for it, especially if Apple went the route of simply making the screen taller.

“It doesn’t really matter how tall it is for us, it’s probably a bigger deal for games . . . I think that a lot of the things we do is just list screens [of ticket information], so having a taller screen makes sense,” he said.

But lowering the pixel density to accommodate a larger screen? The screen is the most important part of the iPhone, said Dinch: “If they augment that in any way negatively, that would be insane.”

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Analyzing the wearable computing market
• Defining the mobile wallet: what it is, why it matters
• The future of mobile: a segment analysis by GigaOM Pro