Here is a quick drawing of a combustion chamber looking at it from the top.
My understanding is, that if you are about to make an "8 inch system", then the round duct that goes through the bench will be 8 inches in diameter. The area of that hole would be (pi * r * r) pi * 4 * 4, or pi * 16: 50.27.
So the area of the hole for the woodfeed and insides of the combustion chamber must be about 50.27. Since it is square, then it would be "the square root of 50.27": 7.09 inches. So it should be about 7.1 inches by 7.1 inches.
In this picture, the dark blue is steel. The gray is insulation. The red is the combustion chamber and the orange is the exhaust that travels between the internal combustion chamber and the outside barrel.
My understanding is that the orange stuff must work out to be at least 50.27.
So there are two circles. One (according to my tiny grids) is about 17 inches. And it looks like about a 3 inch gap. So the barrel has a diameter of about 23 inches.
So the outer circle area is 23 * 23 * pi: 1662.
The inner circle area is 17 * 17 * pi: 908.
The area of the donut is 754. LOTS more than 50.27.