Reducing barrel size

I am planning a rocket mass heater for a green house. I'm more interested in heating it slowly and storing the heat. Does anyone know if it is possible to reduce the size of the "barrel" and still have a full size system function properly?

I don’t think you can reduce the system size heat riser  on a J tube design but you could use a riserless design like a Vortex stove or a shoe box design  then you don’t need a barrel at all.

Hi Jack;  
Welcome to Permies and welcome to the wonderful world of rocket science!
As John said you can completely eliminate the barrel if you use a riserless core.
However there are other options.   A brick bell rmh batchbox has no barrel at all , heats slowly and maintains a slow release heat.

I have had a greenhouse /art studio here in northern Montana, heated for the last 7 years with a rmh.  For a long time we used an 8" J tube and a long piped mass. The barrel was thickly covered with cob for 3/4+  of the barrel.  Now I have upgraded to a 6" batchbox with the same piped mass. I still use the same barrel but now in place of cob I have the barrel surrounded with brick to within 6" of the top.
Either design has kept our greenhouse well above freezing 40-55 F with no fire from 9 in the evening until 7-9 am when I relight. We commonly are in the teens overnight and occasionally below zero.
Our building is 12 x 20 long with a 12 x12 pitch roof 16' tall . All plastic and glass with no insulation in Montana, in the winter, with no fire for 12 hrs  is darn impresive!

What design RMH were you thinking of?
My Auto shop has a hybrid 7" batch into double barrels and then into a large brick bell.
I could have easily made it into a full brick bell with no barrels at all.  
I wanted /needed the vast quick heat of the double barrels but I also wanted the long term heat stored in the brick bell.