Many of the Broan/Nutone ceiling ventilation fans are designed for a 6" duct; here is why.   The American Society of Heating, Refrigeration and Air Conditioning Engineers has released a new standard saying a six inch duct is superior and in many cases a 4 inch flex duct should not be used at all.

For example with a 4 inch duct, a 80 CFM fan designed for 4" ducting can only use a maximum of 3 feet of flex duct or 35 feet of rigid duct. (Deduct 15 feet for ever elbow).   Flex duct cannot be used at all with a 100 cfm fan.   With Broan/Nutone 6" duct fans, the Broan QTXE 110 CFM fans produces full power at an incredible 113 feet of flex duct.

Popup details for each fan specify the design duct size.