I would recommend checking with the manufacturer of whatever unit you are considering/using. Ask them or look in a manual to find the recommendations and spec's for stack diameter and height. That will be your answer. One huge issue with OWB's and stacks is that generally speaking, the taller the stack the greater the draft generated through the unit. This can and often does lead to severe over firing conditions especially in a natural draft appliance like the CB in the picture. Too much draft can be worse than not enough.