From Apple's diagonal measurements [1], it looks like the screen is actually a few mm off from the closest ISO 216 sizes.

d = 7.58in = 192.532mm [1]

d^2 = w^2+h^2

Unfolded:

h = d/sqrt(3) = ~111.1584mm

w = h*sqrt(2) = ~157.2017mm

Closest ISO 216 size: A6 (105x148)

Folded:

D = 5.36in = 136.114mm [1]

H = D/sqrt(3) = ~78.5855mm

W = H*sqrt(2) = ~111.1366mm

Closest ISO 216 size: A7 (74x105)

[1] https://www.apple.com/iphone-duo/specs/