HI Doug, You cannot build a Simple-Fire charcoal generator too big. Now, you can make the nozzle too big, For instance, if your charcoal gasifier was made from a 500 gallon tank and you kept the air inlet set at 1" it will work fine. Be forewarned that it will take some time at start up to purge all the air from your large gasifer and have it replaced with carbon monoxide. Now if your nozzle is too big, say 6" in diameter and you are using a 20 HP engine to pull suction, then the oxidation zone will be too big, and not get hot enough to reduce CO2 to CO.
If you plan on a 50% de rating of your generator output, you will not be dissapointed. It is easy to give a little more power to your charcoal gas by dripping some fluid in the air intake.
As far as engines, I purchased a $600 harbor freight 7KW. This is the Predator brand and is easy to replace the air filter with a charcoal gas inlet. I've only put about 30 hours on my unit, but it starts up easily.
You are correct about charcoal gas not gumming up you carb. However, I will recommend you remove the hose, open the filter and basically let any moisture in the plumbing evaporate. This will cut back on any rust or corrosion of aluminum.
Gary in PA