Why will it not sell ?
Big phones don't sell ? of course they do.
They just sell to an focused audience on "bigger is better"
The dissociation of screen size and quality will still be present if the iPhone 6 is more powerful than 6c , despite having a smaller but perfectly usable screen.
This way the bigger screen will be less usable, less performant but still a full fledged BIG iPhone.
You have notebooks with bigger screen than 15 inch, more storage options...