If we assume that the iPhone 12 would have the same aspect ratio as the iPhone 11, and that the "margins" between the edge of the device and the edge of the screen will be the same as well, we get:
iPhone 12 (5.4") is 2.69x5.30"
Body size is 26.67% larger than the iPhone SE
Screen size is 62.27% larger than the iPhone SE
That is only 1.3% smaller than the iPhone SE 2 released today. This device size is here to stay.