anyhow .. will be interesting to see from someone who bought one,
whether it is different from the rear rack mounted
Ortlieb Downtown bag which has specs of being:
36/14.2'' x 36cm/14.2'' x 14cm/5.5'' = 18L/1098cuin.

leveling the rack mount could end up with something that you could hang
from the top rail of a porteur front rack
FWIW,
Brompton's front rack for M and P bar bikes is 40cm x 30cm,
(the base of my Brompton 09 Touring bag is 20cm deep)

Todd at Clever says O bag 'per the site: 20 litres capacity.
50cm x 30cm x 15 cm high' [must be laying down,high = depth..]
standard frame 40 x 30, so hare wider , bit shallower..