Ya learn something new every day.
I've not heard of a Kawasaki accessory tank bag for the C10 before.
I believe that they made one for the C14 but that was dropped due to poor sales due to the usual manufacturer idiocy of pricing it at 3x the price of the best of the competition.
Looks like it was USA only so hopefully someone your side of the pond has seen one before and has some info.
Otherwise all I can suggest would be to measure the tank bag and use the measurements to calculate the approximate volume of it.
If it's 40cm long, 25cm wide and 20cm deep then it's (40x25x20)/1000 = 20 litres.