If you are building your own range hood, you would want to make sure that you size your ducting such that the flow rate is at a certain optimum (within certain limits) so that noise and unacceptable friction losses are avoided. I will not reproduce data that the engineering toolbox has done very well. This page focuses on determining the CFM and not the ducting. However, if you’re buying a range hood follow the instructions in the manual for the dimensions of your ducting.