OK, at least I did a measuerement. Hope this will help the other owners of the Universum Supersound 16000: You need 3 belts:
one flat belt, 230 mm x 2.5 mm x 0.5 mm (perimeter (lenght) x width x thickness)
two square belts 120 mm x 0.8 mm (perimeter (lenght) x width)
These are the sizes of the old, used belts, so take them a little bit smaller in perimeter. I will try to order these belts and write down here, wether they work properly.
regards, Marco