By default, Design Master HVAC sizes to even ducts sizes automatically. To size ducts to odd sizes, go to DM HVAC->Customization->Options->Edit Project List and change the Ductwork->Use odd round duct sizes below this size option. 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″ (for example, 5″, 6″, 7″, and so on) and even numbers above 12″ (for example, 12″, 14″, 16″, but not 13″).