Rathores blog

Hmm the “iPhone Math” or more likely the iPhone+ or iPhone Plus. Some think it’s the code name or temporary moniker for the rumored low-cost iPhone, but Marco Arment thinks it’s something different. He thinks it’s a super-sized iPhone close in size to the Galaxy Tab Note.
And I think Marco might be onto something.
According to his math if you did something like a Retina iPad screen with iPhone 5 design?

The theory is easy to understand: perform John Gruber’s Mini-predicting math backwards. The iPad Mini uses iPhone 3GS-density screens at iPad resolution. What if an iPhone Plus used Retina iPad screens with iPhone 5 resolution, keeping the rest of the design sized like an iPhone 5?
Its 640 × 1136, 264 DPI screen would measure 4.94″ diagonally, and it would look roughly like this next to an iPhone 5:

                         
An iPhablet?
Why not. Really.
I don’t think anyone could have predicted the huge uptake of both tablets in general (especially as near-replacements for laptops) or even phablets. Apple, I think, sees different sizes of tablets and phones as an asset not a distraction to the iPad/iPhone line. Or at least with the runaway success of the iPad mini they probably do.
Marco points out that a couple sizes for all tasks might not work, but several sizes might:

An iPhone Plus almost as big as a Galaxy Note isn’t ideal for many people, but it doesn’t need to be quite that large to accommodate a 4.94″ screen. It’s clear that other manufacturers have found designs and techniques to make larger-screened phones require smaller bezels. Apple could achieve similar results and shrink the “forehead” and “chin” even further, limited primarily by the size of the Home button and the desire to keep the forehead and chin equal height.

Having a device that large might stay in your bag most of the time (when you’re not at a desk or something), but interact with other wireless devices for calling and seeing incoming messages. Frankly, I think we’re only beginning to understand how powerful tablets fit into our computing lives.