Senior analyst at Digitimes Research Luke Lin has been keeping an eye on Apple's supply chain and believes he has the floor plan of the next generation iPhones. He divides them by screen size and type, but memory capacities also play a role. The successor to the iPhone X will stick to an OLED display of just about the same size - 5.85". However, Apple managed to get the bill of materials down 10% from the $412.75 of the current model. The OLED display accounted for a big chunk of that. Which is why there was a 5.85" LCD model in development, but that was paused for the Lunar New Year...
Source: https://www.gsmarena.com/digitimes_iphone_x_successor_to_be_the_cheapest_model-news-30191.php