Can my iPad open iPhone apps using the iPhone 5 form factor? At the moment, my iPhone apps(for example: Yahoo Weather) on my iPad 3 Retina running iOS 7.0(11A465) are displayed using the 3.5" iPhone screen in the iPad, is it possible for me to change it to display the 4" iPhone screen(iPhone 5) in the iPad?

Thanks.
No, this is currently not possible. iPhone apps running on the iPad will use the smaller iPhone 4s screen's aspect ratio.