Specifying ductwork size to conduct an analysis

Does anyone know of a way in which to specify ductwork sizing within the system builder.

I have been tasked with analyzing an all ready existing system. I have floor plans etc so can create the building etc. The only thing I cannot do is set the ductwork to the dimensions specified by the HVAC contractor, who has done an on site recce in lieu of me. 

All i have managed to do is draw the ducts as per the recce report, and i have sized the system accordingly to the details I have been given, but the ductwork sizes are different. Can I manually change the system sized ductwork?

Thanks,

Duncan