I don't like I phones, so making a bigger one would suck. If I want a big screen, I'll get a tablet. Phones aren't supposed to be really big. They're supposed to fit in your pocket. I'd take an Android over an I phone anyday because I don't like Apple.
Yes! A bigger screen would make me much more likely to buy an iPhone. Currently the iPhone 5S has a tiny 4" inch screen while many android phones are hovering around 5", which makes them much more enjoyable for media consumption, and browsing the web. A larger screen would persuade me, and many others who love the larger screens of android devices.
The iPhone is expensive. A bigger screen can be nice, but unless someone already wants to buy one the screen size isn't going to make them more likely to buy one. Better features, a lower price, and better functionality would all be more important improvements for the iPhone and would help sell them.
As a matter of personal choice I have never considered owning an iPhone due to the size of screen. But with a larger screen I would consider purchasing an iPhone, but the larger screen is not a deciding factor in my choice to buy the iPhone. It would be considered along with all the other pros and cons of owning the iPhone.
A major flaw with the iphone, is how easy it is for the screen to break. It doesn't take much to render the phone useless, and unless you have insurance, you're going to be charged a ridiculous price to get it replaced. While a bigger screen would have many benefits, it also sounds even more easy to break.