It's rumoured to have a 4.7-inch screen, so probably not much smaller than the iPhone 6,7 and 8, even if they are able to reduce the top bezels somewhat.
a 4.7 inch screen with similar bezels to an X would be about .1 inches smaller than an 5S/SE.
According to who?
"Apple is also working on its first low-cost iPhone since the iPhone SE. That could come out as early as the first half of 2020, the people said. The device would look similar to the iPhone 8 and include a 4.7-inch screen." -- https://www.bloomberg.com/news/articles/2019-09-05/apple-wor...
According to gsmarena.com iPhone 8 is 4.7" and SE is 4.0"