So, it seems safe to say that the 2020 iPhone with the 5.42" screen will be larger than the iPhone SE. But, maybe not by much.
Making the assumption that Apple keeps the 19.5:9 aspect ratio (which I think is a decent assumption, but by no means a lock), a screen of that size would have a bounding-box of 4.92" x 2.27". Compare that to the bounding-box of the SE at 4.87" x 2.31", and the height is already over while the width is slightly under. Once you add bezels, however, the new phone will obviously exceed the SE in both dimensions.
The big question is, will the 2020 iPhones increase screen-to-body-ratio from today's models? We can optimistically say that the answer to this question is probably "yes", but by just how much would be nigh impossible to accurately predict at this point.
Best case, bezel reduction is very high and we get a device that is so close in size to the SE that it essentially matches that phone in pocket-ability. Worst case, we could be looking at a device that is approximately 1/4" inch wider and taller (1/8" all around) than an iPhone SE, which would still be a nice size for a compact phone, but would lose appeal for those looking for an SE-sized iPhone replacement.