We've heard rumors of this foldable screen smart phone coming from Samsung.. and they unveiled it yesterday at their 2018 developer conference in San Francisco. It's still in the prototype stage, but Samsung jumped at the chance to show it off.
The screen size doubles to almost the width of a tablet with a 7.3 inch display when it's open, but it is the size of a typical phone when folded. It's said to be due out next year..