Sizing for Greenhouse

Does anyone have any data on how to size a rocketmass heater for a greenhouse? In my instance 18X20 in zone 7.
David

Dave; I use an 8" in my 12x20 greenhouse, with a raised mass apx 2.5 w x 2.5 T x 15' long. I Use the mass to set the for starts on to keep them warm. There is a post just for rmh and greenhouses , that may answer your question. A copy of ianto evans book will also answer a lot of your questions.