A: The duct is slightly larger than 30 inches to allow it to fit over a 30 inch fan. It is the same diameter all the way across.
A: This duct can extend up to 128 inches.
A: Hello Monsi, unfortunately we do not have a 20" duct available at this time.
A: Hello CraigS, unfortunately we do not manufacture a collapsible duct for the 42" drum fan at this time.