The first question when considering an RMH is, what are your heating needs? How big is the space you want to heat, how insulated, and what is the climate? If you are in the Sierra foothills in California, you presumably don't usually have a very harsh winter. Thus, unless you are trying to heat a large house, a 6" system may be right, which just happens to be the largest size that will conveniently work with a 30 gallon drum.
The standard recommended proportions for a J-tube core are 1:2:4, feed tube height to burn tunnel length to riser height, all measured along the outer edges of the cavity. So you might have, for a 6" system, 12" feed, 24" burn tunnel and 48" riser. This will want its fuel cut to 12" lengths, for best control of the fire. A 6" system will have round sections of flue 6" diameter and square (brick-built) sections 6" x 6" in cross section. The burn tunnel can be a bit less if necessary, but not larger.
The riser will be 6" diameter inside, and with common inexpensive construction, at least 2" thick so as not to lose too much heat; 10" outside diameter total. You can make it thicker, but I don't think there is significant benefit in going from 2" to 3" insulation thickness for a 6" system. You want to support the barrel so that there is a 1 1/2" to 2" gap from riser top to barrel top. More is okay; the closer the gap, the more heat will be concentrated in the middle of the top. This may matter if you want to do much cooking on it, but if that is not important, you can make the gap larger.
I posted a couple of pictures of a 6" system I built a few years ago in this thread: https://permies.com/t/40/53224/Portable-Compact-RMH-Hybrid-internal#442157