Apple's 2019 iPhones will be the same size as its current iPhones, according to a note from TF International Securities analyst Ming-Chi Kuo as reported by 9to5Mac. Kuo is generally considered to be a reliable source for Apple product news considering some of his predictions have proven correct in the past.
It's also possible that Apple will use organic light emitting diode (OLED) screens for all three of its 2019 iPhones, including the least expensive model. That would be a departure from the company's current lineup, as the entry level iPhone XR is the only device in the newest iPhone family to use a liquid crystal display (LCD). OLED screens typically offer deeper blacks and bolder colors, resulting in better contrast than LCD displays.
South Korean news source Electronic Times reported that Apple may use OLED screens for all three of its 2019 new iPhones in May, but Kuo predicts that one model will still have an LCD display.