The size of the screen on the original version of the Apple iPhone was 3.5 inches, which didn't change until Apple went to 4 inches on the Apple iPhone 5. The screens on both of the models released last year, the Apple iPhone 5s and Apple iPhone 5c, were 4 inches in size.
Apple feels the pressure to raise the size of the iPhone screen after Android manufacturers have raised their screen size to the point where a 5 inch display is considered normal. In China, 40% of Android phones purchased in 2014 had screen sizes exceeding 5 inches. With the iPhone producing $91 billion in sales last year, it is Apple's number one product, which means that the company is under pressure from flagship Android models that come with screens in the 5 inch to 5.5 inch range.