The much rumored 5-inch iPhone is no exception as the folks at MacRumors have created pdf that, when printed at 100 percent scale, gives a pretty accurate representation of what such a device feels like in the hand.
While there are multiple theories floating around as to how Apple could scale iOS for a 5-inch iPhone, the most probable one as of now seems to be Apple maintaining the same iPhone 5 resolution of 1136 x 640 pixels and reducing the pixel density to 264ppi, which happens to be the same as the display on the Retina iPad.
By printing this 2.4 MB PDF at 100 percent scale, and subsequently folding it to create a three-dimensional dummy model, you can get a reasonable feel of how big a 5-inch iPhone might feel in your hand. You obviously won’t be able to navigate the entire screen using just one-hand, but you could test to what extent would this be valid. Additionally, you can also see how existing apps would appear on the larger display by printing this PDF.