Following construction principals in some of the library books and using one or two sheets of paper makes more interesting results than random guessworking…
Everything starts with the motor’s displacement and RPM, “rounded” by the degree of load.
All critical flow areas are in direct proportion to the calculated gas demand.
That applies to the process volumes too.
The practical formulas are plain multiplications and divisions, and a few calculations made by internet formulas only needing an input of diameters and height.
Then the fuel size will also appear as how many bits needs to fill the upper hearth = oxidation zone.
The only formula on the internet needed is a “conical frustum” calculator, which will sort out the measures for a needed hearth volume.
Do you have a “case” to sort out?