Duct sizing is an important, but often poorly executed, aspect of HVAC design. But it doesn't need to be complicated. The ASHRAE Fundamentals manual discusses the Equal-Friction method, which is the method I most frequently use. To cite the manual: In the equal-friction method, ducts are sized for a constant pressure loss per unit length.
In the video above, I demonstrate how to implement this duct sizing method on a simple project. The duct calculator (ductulator) shown in the video is available for free by following THIS LINK.