Size question

I would like to use RMHs in small insulated buildings, roughly 8'x16' and 16'x24'. The dvds and E&E's book don't say much (at least that I've found) on smaller spaces. Would it be better to drop down to a 5" dia duct or just shorten 6" ducting or am I overlooking something? My goal is to get the mass to where it will keep the place warm for at least 18 hours, before melting the occupant with the heat off the barrel. Any thoughts or suggestions will be appreciated! Thanks!

It sounds like you are planning a J style rocket.
If so,6" is the smallest size recommended.
Smaller J style  rockets tend to be finicky.
Don't worry about it being too big, the thermal mass dampens the extremes of the heat.
In some ways, the rocket is like a bucket used to fill a bathtub(the thermal mass).
The bigger the bucket, the faster you can fill it...
I would consider putting cob all the way up the sides of your barrel,leaving only the top bare.

Thank you for your suggestions William. Covering most of the side of the barrel sounds like a good deal!

Cover your barrel no more than 2/3 or so. Cob, nice and thick at the bottom and getting thinner as you near the top ring. The barrel will expand and contract and your cob covering will have a small gap visible at the thin area.  Use plenty of sand in your cob mix or you will get excessive cracking.

Thomas,    I've always liked the idea of cobbing up around the barrel to decrease immediate heat release and increase mass in my small shop and also to reduce the fire hazard of my wood work bench so close to the barrel  but have held back on that one until I come across a barrel that has a detachable lid with a band to hold it in place. Yours appears to have one. Cleaning the top of the riser and around the barrel can be a lot more difficult otherwise.

Hi Gerry;  I purchased my barrel at  the north 40 farm and ranch store. I think I paid $30.00 for it.  Having a barrel with an opening lid is essential to perform maintenance and inspection.
My RMH is in a greenhouse/studio, my core is all heavy firebrick and the barrel has as much cob as I could get away with. I wanted the most mass as I could to keep the heat in the room. After 5 years I can tell you IT WORKS ! We went from 12 + cords of wood to less than 5 !  Before we had to keep a fire going all night ... now we have NO fire all night,in a plastic greenhouse !