With the OLED redesigns they're not making the physical sizes smaller, unfortunately. I thought that would have been a great opportunity for them to do so.
This is the first rumor they have had
recently about next year's (2019) display sizes (
https://forums.macrumors.com/thread...ay-sizes-in-2018.2046854/page-2#post-24601220), however I remember previous rumors providing similar numbers, so that's definitely what will happen.
4.7 inch iPhone will be phased out, same physical case size, but 5.3 inch display. Plus model will have the same physical case size, but increased display size of 6.5. SE model may get a display update to 4.3" or so, if I remember correctly from previous rumors.