By default, Design Master HVAC sizes to even ducts sizes automatically. To size ducts to odd sizes, use the Options command.
Ribbon: DMH: Customization->Project Customization-> Options
Pulldown Menu: DM HVAC->Customization->Options->Edit Project List
In the Ductwork section, find the Use odd (shape) duct sizes below this size options. Ducts below the specified size will be sized to even and odd numbers. Ducts above the specified size will be sized to even sizes only.
For example, if you change the value to 12, ducts will be sized to even and odd numbers below 12″ ( 5″, 6″, 7″, and so on) and even numbers above 12″ (12″, 14″, 16″, but not 13″).