That is certainly true (Craig), but when you have a chimney that is drawing well, it will accelerate the combustion in the oven by "scavenging", i.e. removing combusted air that will be refreshed by new oxygen rich air. Optimal is 15% of the opening, modified by the height of the stack. The taller the stack, the smaller the cross section of chimney required. The shape is important as well, with round optimal, square next then rectangular.

To calculate usable area of a chimney draw a circle(s) in the cross section such that the circle diameter is the minimum dimension of the flue. That is the usable area of the flue, and as stated, it should fall in the 15% of the opening area to be most effective, since stacks on ovens seldom fall into the height needed to modify that down.

In short: Make it as big as practical and fire it up.