Daniel - As mentioned, you can find heating requirement calculators online. Once you have a number,
batchrocket.eu has information on system sizing for the requirements, for batch box designs. J-tube designs need to be scaled as mentioned above, with an 8" J-tube being similar to a 6" batch box. When you do have a system size determined, all the chamber proportions are determined by that, and described in numerous sources including batchrocket.eu. In general, oversizing a system a bit only means that you need to run it less often; if you undersize the system a bit, you need to run it more times a day in extreme weather which may be inconvenient. Greatly undersizing can be a real problem.
Self-feeding experiments have been done by many people, but they are all experiments. I would not advise trying to do such things in a system you want to just work out of the box. Also, self-feeding implies continuous or long-term burning, which is not required by the concept of a fast, hot fire that heats a mass for continuous gentle heating. I recommend you go with mainstream designs, and with a bell or stratification chamber rather than piped mass, to make your build as reliable as possible. To be clear, if you do not have unusual conditions, recent common designs can be expected to work reliably if followed. As far as specific layout, proportions and surfaces of your installation, those can be tailored to your situation as long as you follow the basic technical rules.
Especially anything to do with history, geology, archaeology, paleontology...
