Cody, this looks to be a wood gasifier. I am afraid it may not work as a charcoal gasifier as it might burn out. The design is excelent but l wuld guess if you want this to work safely with charcoal the pipe wuld have to be at least about 8mm thick. I dubt you can roll that thick. There must be enaugh thickness to conduct the heat away from the nozzles and in to the incomeing air.
The thing is it doesent realy matter the HP or engine size, the flute style nozzle (wich this kinda is) needs to be of a minimal size to work, l wuld say 2" dia is what l aim for.
As for the design of the rest of the gasifier, when you get this nozzle ring built, you are preety much done. I wuld keep it as simple as possible. The downdraft charcoal gasifier does not need a restriction. Just a grate. This means the firetube is allso not nessesery.
What you can do is this
Sorry for the roughness you can fix a detachable grate in the 55gal drum, at the bottom. Only weald the pipe going out. Above it you put the nozzle ring. If you have it, put a cheramic wool blanket to shape the firetube but if you dont, mineral wool moistened with sodium silicate will allso work. Over time, even ash will and fine charcoal dust will naturaly form a firetube.
Needless to say, this will greatly simplify your build. Less seals equas less air leak potential! Allso the extra large ablmount of insulation around the firetzbe will keep the heat where its most neaded.
As for size. Depends a lot of what size fuel you plan on useing. If we talk about engine grade charcoal l wuld say the internal diameter of the ring (nozzle to nozzle) shuld be around 8" with the height from nozzles to grate at say 7" or 8" for your engine. But generaly the answer to your question is no. A charcoal gasifier generaly has a smaller glowzone because the heat is higher (makes no sence at first does it )
One more thing. Do make the air supply come in tangently on the ring. I had a gasifier fail on me because air slamed in the nozzle opposite to the air intake, making it overheat. A tangential spining supply of air will asure equal distribution.