Maybe a better way to ask the question would have been: How long can prunings be for use in a
rocket mass heater?
To
answer some of your points, there a difference between a fire that right's next to you (say by my work desk while I'm working) and a fire that's not close by. For example, when I'm in the room I keep the glass doors open on our fireplace (but the mesh curtain closed). When I step away for a little while, I shut the glass doors, just to be extra safe.
We can't have anything dangerous. But there's still the question of saving time
cutting wood by not making the fuel too short.
Lil side question: one shouldn't use wood with bark on it in a
RMH? What with the temperatures, I'da thought it'd be ok.
Thanks,
Ari