Ratios for a J-tube core: 1:2:4 feed tube to burn tunnel to riser. It is common for the "1" base to be 1 1/2 to 2 times the system diameter. You can measure the lengths either along the centerline of the core, or along the outside edges.
In any case, it is best to make the core cross section squarish, and if not square, the height should be greater than the width. The cross section should not vary throughout the system, except at a couple of transition points where you want it larger so as not to restrict flow.
I don't think the L-tube is as exacting with regard to proportions, but I would stick with Max's ratio. You want enough length that most of the gases have time to combust before reaching the top of the riser, but not all (if you are planning to cook on it, which is the typical use for an L-tube). Testing mockups outdoors would be the best way for you to find out what will perform best in your situation.
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