The ideal diameter for aluminum flexible duct is determined by the specific airflow requirements of the HVAC system and the size of the equipment. Larger buildings or systems handling high-pressure air will generally require larger diameters, while smaller-scale applications may use smaller ones. Common diameters range from 4 to 10 inches or more, but the appropriate size should be selected based on the precise dimensions and air pressure needs, ideally with professional guidance to ensure efficient operation.