HVAC duct sizing can seem like a challenging task. If the duct is too small, air velocity will be too high which will cause increased restriction, decreased airflow and noise in your HVAC system. If the duct is too large, air velocity will be too low which will cause premature warming/cooling of the air and low airflow through your HVAC registers. However, with the proper resources, HVAC duct sizing is a fairly easy thing to determine.
Purchase a Ductulator. Trane makes a wonderful one that retails for around $13.00. (See Reference 2.) As a lower cost alternative, there are a few websites out there where you can perform the same calculations. (See Reference 1.)
Figure out the CFM (cubic feet per minute) you want to flow through your duct. You want to maintain your airflow between 600 and 900 FPM (feet per minute). Higher than that will cause noise and decreased efficiency. Lower than that will cause premature warming/cooling of the air and decreased efficiency.
Using the Ductulator, or online calculator, match the CFM requirement to the friction rate you plan on using. Standard friction rate is 0.1" water column per 100 feet. It will then give you an equivalent duct size in both rectangular duct and round duct.
Purchase and install the duct size that is given to you by the Ductulator or online calculator.