The standard recommended ratios are 1:2:4 for feed tube, burn tunnel and heat riser.
But if you think you are going to build a permanent
RMH using a steel combustion zone, think again. As some like to say "Metal is doomed!" Even heavy steel will be damaged by the heat and atmosphere, and corrode and/or soften and buckle (often near the base of the riser). An 8" diameter system will generate extreme heat and fail faster than a smaller system.
Also, the steel will conduct heat away from the combustion, cooling it prematurely and lessening efficiency, unless extremely well insulated. (This last part is less significant for an 8" system than for smaller systems.)
