Like I mentioned in the previous article on iPhone 5S's hardware features, bigger screen iPhone would be a logical step to take for Apple, based on the popularity bigger smartphones have been gaining. But as Rene points out Apple tends to looks at consumers inherent problem than the solution they think is the right one for them and solve the problem in a unique Apple way, like the iPad instead of Netbook.
Also refer to this article that Rene posted sometime back on the dynamics of a larger iPhone, lots math there, but very intuitive. But I stick to my point that even though I would like a bigger iPhone, probably 4.3" or 4.7", I would not want it at the cost of one hand usability. Or can Apple do something like reduce the bezel sizes (like they did for the iPad Mini) to make a bigger screen with one hand usability? Unlikely, but it is fun to speculate!