After the recent debut of the massive iPhone XS Max, The Economist made this tongue-in-cheek graph showing progressive growth in the size of each iPhone model over time. Extrapolated into the future, it suggests that iPhones will be almost as big as the iPad Mini tablet by 2025.
Obviously, iPhones can't get too big, or else they won't be able to fit in standard-sized pants pockets. However, that doesn't mean the trend will stop. I think it means an iPhone with a foldable interior screen will be unveiled in the 2020s. In fact, Samsung is already working on such a device:
If we envision folding phones as being structured like simple manilla folders, the phones will have two screens:
1) A relatively small, non-bending screen on the "front cover' of the manilla folder.
2) A large, flexible screen spanning the whole interior of the folder.
Users will carry these phones "folded up" most of the time, and will primarily use just the non-bending "front cover" screen. On rarer occasions, they would unfold the phones and use the larger, inner screen.
Foldable smartphones will make mini-tablets obsolete, and probably eat into market share for the standard-sized tablets as well. The popularity of tablets has already stalled.