Double diameter riser

Hi all, i'm in the process of designing my own rmh, a 230mm one.
I have decided to make an octagonal brick riser, with waterjet cutted bricks, but due to the cost i'm thinking to partially build it with store bought refractory chimney elements (1000°C continuos working temperature).
So the first 500-600mm will be stacked bricks the remaining 1000mm or so it's going to be made out of tubular refractory.
The problem is that i can only found such elements in 200mm or 250mm diameter.

Attached some pictures of my drawings:
Note that the p channel is going to be integrated in the structure of the core, everything will be made with high quality bricks, each of them will be slotted (like a lego brick).

Hi Matteo;
You can increase it from 230 mm to 250mm with no problem.
Do not decrease down to 200mm, there would be flow issues.
Be sure to use no less than 230mm for the final exhaust stack.

Very good, i have another question, regarding the correct sizing of the rmh for my house. I have used the excel file to calc the diameter of the riser needed, but It seems to me that the power output is off by a good margin, could it be?
A rule of thumb of wood burning stove is about 3kg of wood burned in an hour is equal to 8kw/h, in my case with a 230mm riser and 21.6 kg of wood the output should be over 15kw/h, not 7.6 like stated in the excel sheet.