I do not have any sage words of advice to offer. My dry stacked test burnings have been in a 4.5" system. That's because the fire brick I bought measure 2.25 x 4.5 x 9.0 inches
I'm too early in the process to give any advice, but I can say that it draws and burns as I have read j-rocket stoves behave. Temperatures seem to be in the right ball park, for what I am able to measure. But this is only a mock-up to allow me to test the basic principals and obtain heat estimates. For my instance I need to keep weight low, so I'm starting small, and will see where that takes me. And I have no idea if it would have enough "thrust" to push the exhaust through a thermal mass heat exchanger.
The only other thing I could add is to briefly describe where I'm at, and if that's of any use to you, great.
This is outside and dry stacked fire brick. I have a big pile of sand (for
gardening). I shovelled about an inch of sand on the ground, then laid two 2x6's on top of that, and shovelled about another inch of sand on top of that. This was to 1) provide a somewhat level working surface; 2) allow me to stick a thermometer probe into the sand below the fire brick and read the temperature there; and to give an opportunity for the 2x6's one inch under the fire box to burst into flames (I say only half-joking).
I have since built a short ring of block and brick around the perimeter of the fire brick, and shovelled more sand into this. This helps insulate the horizontal area of the J. It is also the base for my support onto which I'll place my 55-gallon barrel (which is the step I'm at right now).
My current build has a layer of fire brick face down on the sand bed, and this is the floor of the fire box. Then I stacked fire brick on edge making a perimeter U-shape, with the open end at the
feed tube. The internal horizontal measurement is 16.25" (D). At the end that I built the feed tube, the fire brick that enclosed the perimeter is stacked with its long axis vertical. That way I can lay the third
course (that also forms the bridge/roof of the burn chamber) on edge, and have the top of the feed tube opening at the same height as the top of the bridging bricks that form the roof of the burn chamber.
At the opposite end -the fire riser- I make the normal on edge square risers, with the fire brick. That height, measured from the roof of the burn chamber is about 36" so it is a little greater than a 2:1 rise (E : D).
The maximum temperature I have measured under the burn chamber is 175*F, and that happens after I rake the coals to sit above that spot (the coals are separated from the thermometer probe by the 2.25" thickness of the fire brick floor and about 1/2" of sand).
The maximum temp. I have measured at the outside-bottom-rear of the fire riser is 225*F. The probe was pushed down through about 8 inches of sand, with the probe against the outside of the fire brick, so it was separated from the internal flames by the thickness of the fire brick (2.25").
I do not have a thermometer that is able to read the exhaust temp. out the top of the fire riser. All I can say is this very quickly ran past the 550*F limit of my thermometer, showing no signs of slowing down. A cheap magnetic "flue" thermometer attached to a 6-inch piece of stove pipe I sat on top of the fire riser (to hold the magnetic thermometer) displayed about 225*F. But I have no idea how accurate it is, and it was only $10USD, so I assume it is not highly accurate.
That's about all I know right now