The minimum duct sizes of range hood and the recommended duct sizes of range hood are based on the cubic feet per minute rating of the duct fan. In other words, the duct size depends on the CFM of your range hood.
Generally, the duct size required by the fan hood manufacture and the recommended duct sizes are large than the standard size.
CFM is short for cubic feet per minute (cu ft/min). The rating of cubic feet per minute is a measure of how much air a fan can flows out form a space in a minutes time. The duct size is proportional to CFM rating. So if you use too small a duct, it creates too much drag on the airflow, and it defeats the value of the higher CFM fan. The suitable duct size can ensure range hood as efficient and quiet as possible.
See the graphic below for more information:
The minimum and recommended duct sizes for the hood fan’s CFM rating are as follow: