More energy is lost through improper duct sizing than any other part of the system. Inefficencies in design and size can greatly effect the performance of the overall system. Proper size is key to get proper air flow into the home. Coming up with a plan is a must before installing ducting. Without a proper plan ducting can be stretched to fair, bent to much or forced into narrow areas that constrict airflow.
The Air Conditioner Contractors of America (ACCA) manual "D" is used as a standard in calculating duct sizing. Insist of a duct plan before installing your ducting. Listed below is a standard checklist for residential properties. Reference the list to know what questions to ask when having your duct size determined.