alt Hacker NewsFrom 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)
I imagine that would be to account for the bezel. The outer display is slightly less than half the size of the inner display, so to get the same aspect ratio they would have to adjust it slightly.