The iPhone 15 Plus has a 6.7" screen size, just like the iPhone 15 Pro Max. The rumors we've heard so far regarding the upcoming iPhone 16 Plus mentioned it will keep the exact same screen diagonal as its predecessor.
But things will change next year, according to a new rumor. The iPhone 17 Plus is apparently going to have a smaller screen than the iPhone 15 Plus and the iPhone 16 Plus, Ross Young of Display Supply Chain Consultants says.
He doesn't mention a specific size for the iPhone 17 Plus' panel, but does say it will make the phone sit somewhere in between the vanilla iPhone 17 and the iPhone 17 Pro Max. Then again, if the iPhone 16 Pro Max grows to 6.9", then the same can be said for the iPhone 16 Plus too.
It's far too early in the iPhone 17 family's development cycle for us to take this very seriously at the moment, but sure - it could happen. We're waiting for some actual size numbers, at a minimum, in the future, to take it more seriously. That said, Ross Young has a generally very good track record sharing this type of information, so don't be surprised if next year Apple will use sizing to further differentiate its smartphones.