Apple did want to launch the iPhone 6 with a 4.8 inch display but they ran into some manufacturing issues. They did plan on releasing a larger iPhone towards the end of the year but the suppliers of the display struggled to increase the size of the display from that of 4 inches to 4.8 inches.
Apple use in-cell technology which means that the touch display is meshed with the glass into what is a very thin display. Their partners are not able to yield enough displays so they could launch the iPhone 6 with the larger display.
If Apple wanted to release the Apple iPhone this year it would have been the 3rd iPhone for the year according to the roadmap of Peter Misek. A low cost Apple iPhone could be released in June, along with the Apple iPhone 5S, which would come with a slight modification over the current iPhone.
Misek says that there is another reason why Apple will hold back from releasing the iPhone 6.
This is due to the fact that the Apple iPhone will have the 20 nm processor which would means that it could add on more cores. This may be four to eight. Of course to be able to take advantage of the processing speed iOS would need an upgrade. It is thought that this may not be available until 2014.
Tim Cook did stress that the size of the Apple iPhone was right, however Misek thinks that he has been saying this so as to prevent people from not purchasing the iPhone and instead wait for the latest model with a larger display.