According to rumors, the 2020 iPhone Pro will be reduced from 5.8" to 5.4", which could look like this (size comparison including SE): https://blogs.forbes.com/gordonkelly/files/2019/06/New-iPhon...

However that size would still be significantly wider horizontally than the SE, pretty much as wide as the 6/7/8 (which I found too wide for comfortable one-handed use).

In addition, they would probably keep the on/off button on the side instead of relocating it back to the top. The latter would be my preference, also for consistency with the iPads. When I owned both an iPhone 6 and an iPad mini, my muscle memory couldn't quite get used to the inconsistency of the on/off button location.