Info from "unnamed industry sources" claims that the Samsung Galaxy S9 and S9+ will use the same screen sizes as the S8 generation. Samsung Electronics has allegedly already given Samsung Display the specs - 5.77" for the S9, 6.22" for the S9+. That is not to say that the displays themselves will not be updated. If nothing else, Samsung is going to have another shot at placing the fingerprint reader under the screen. Qualcomm and vivo already showed a working prototype, so maybe next year things will work out better (though rumors would have us believe that Apple is struggling with this...
Source: http://www.gsmarena.com/samsung_galaxy_s9_will_reportedly_maintain_the_screen_sizes_of_the_s8-news-26259.php
