I think Apple marketed it as "just right" as it's here to stay. They can't make screens much more bigger without making them a tablet. Anything much over 4" especially width proves hopeless to use as a phone.
I firmly believe Apple will stick to a 4" format for 3 - 5 years atleast.
I firmly believe Apple will stick to a 4" format for 3 - 5 years atleast.