(16*120)/PPI = the length of the area that will be used, the height is 1080/PPI
I think that phone is 443 PPI, so the length would be 4.33" and the height 2.44"
The square root of 4.33^2+2.44^2 = the square root of 18.75+5.95 = the square root of 24.70 = 4.97"
Therefore the 16:9 area of the phone being used for video would be equivalent to a 5" display.
But if you want to know a secret shortcut on this... the P2 was a 5" display, and all they did was lengthen it vertically ... which means the 16:9 area is still 5".