Ming-Chi Kuo states that there will be three sizes of iPhones in late 2020, 5.4", 6.1", and 6.7". That means only the 6.1" size model stays the same size, although it would change to OLED. Note that this does not include the rumoured low cost phone possibly coming in early 2020.
https://www.macrumors.com/2019/06/17/5-4-inch-and-6-7-inch-iphones-2020-kuo/
I'm thinking they would keep the various display point resolutions the same as existing models, but would change the pixel densities:
Small 2020 flagship iPhone
375x812 display points
At x3, it would be 1125x2436
1125x2436 / 498 ppi (eg. Samsung Note 10+) = 2.26x4.89", which means a diagonal of 5.4".
Large 2020 flagship iPhone
414x896 display points
At x3, it would be 1242x2688
1242x2688 / 440.58 (eg. Google Pixel 2) = 2.82x6.10", which means a diagonal of 6.7".
That means they would actually have to decrease the pixel density for the large phone to reach 6.7", although the change wouldn't be much so it's fine. OTOH, they could go with a super high pixel density with a x4 multiplier to 1656x3584, which would be getting closer to 4K, but I think that is unnecessary in such a phone. Or else they could change the number of display points on-screen, but I don't think that is necessary either.
Apart from that, I just hope they can manage lower levels of PWM flickering for those who are sensitive.
https://www.macrumors.com/2019/06/17/5-4-inch-and-6-7-inch-iphones-2020-kuo/

I'm thinking they would keep the various display point resolutions the same as existing models, but would change the pixel densities:
Small 2020 flagship iPhone
375x812 display points
At x3, it would be 1125x2436
1125x2436 / 498 ppi (eg. Samsung Note 10+) = 2.26x4.89", which means a diagonal of 5.4".
Large 2020 flagship iPhone
414x896 display points
At x3, it would be 1242x2688
1242x2688 / 440.58 (eg. Google Pixel 2) = 2.82x6.10", which means a diagonal of 6.7".
That means they would actually have to decrease the pixel density for the large phone to reach 6.7", although the change wouldn't be much so it's fine. OTOH, they could go with a super high pixel density with a x4 multiplier to 1656x3584, which would be getting closer to 4K, but I think that is unnecessary in such a phone. Or else they could change the number of display points on-screen, but I don't think that is necessary either.
Apart from that, I just hope they can manage lower levels of PWM flickering for those who are sensitive.