Dryer vent hoses vary in size, but most used for residential dryers are between 4 and 5 inches in diameter. Using a vent hose that is too small presents a fire risk, while one that is too large can result in inefficient drying.
As dryers heat clothing, the water trapped in the clothing turns into water vapor. If this water vapor is not removed, it simply condenses back on the clothing. Vent hoses rout this water vapor outdoors and prevent the dryer from getting too hot; however, using a vent that is too large can remove more heat from the system than is necessary. In addition, especially wide hoses can cause water vapor to collect in the hose, where it can cool and condense.