40' is about the recommended limit for a 6"
RMH without considering bends, so I think you need an 8" system to power a bench that long. This would make way more than
enough heat to make the space hot, and keep it hot for many hours. Unless you expect to use it every day or so, you probably don't want the thermal flywheel of a full-mass bench; you would probably do best with a minimum-mass bench that lets heat radiate from as much of the duct as possible, with maybe a wooden bench surface with airspaces surrounding the (possibly lightly-cobbed) duct. I might even start with a layer of
cob only on the first bit of duct to moderate radiation to that bench, then add more cob if you decide it needs it for smothing out the heat profile.
Do you have
experience with heated benches in saunas? You might want the advice of an experienced builder/operator to say whether it is really a good thing to have a major source of the heat coming from the bench you are sitting on. It might get dangerously hot in order to make the sauna air hot enough.
