Pipe diameter for rocket stove in a biochar retort

Hi, all.  I'm looking to build a Hookway biochar retort, which basically consists of a rocket stove running through a 55 gallon steel drum that holds the biochar feedstock.  As the feedstock pyrolyzes, the flammable gases are fed into the combustion chamber to sustain the charring process.

The original plans call for using 6" diameter stainless steel (304) pipe, but I have the opportunity to pick up some 5" 304 stainless pipe at a much lower cost.  Given that the combustion chamber is about 1 foot long, and the heat riser is about 3 feet tall, would you expect 5" pipe to still work satisfactorily?

Thanks for your insights.

Hi Curtis;
If you size your J tube dimensions for 5" rocket    4.5"x 4.5"  than a 5" riser will work fine.
Trying with a 6" J tube could work or could run backwards and smoke you out.
Remember  6 pounds of xxxx has never fit well in a 5 pound sack...