enthusedii

iPhone 8 Plus:
5.5" from corner to corner
3.07 by 6.24 inches
19.157 square inches

iPhone X:
5.8" from corner to corner
2.79 by 5.65 inches
15.764 square inches (also, don't forget the notch)

iPhone 8:
4.7" from corner to corner
2.65 by 5.45 inches
14.44 square inches

The measurements are for the whole body not the screen - which means the bezels on the 8/8+ are taking up some of that area. 

iPhone 8 Plus screen: 
5.5" from corner to corner
~2.7 by 4.8 inches
~12.96 square inches

iPhone X screen:
5.8" from corner to corner
~2.43 by 5.27 inches
~12.81 square inches (minus notch)

iPhone 8 screen:
4.7" from corner to corner
~2.3 by 4.1 inches
~9.43 square inches