Flow rate through large air duct

mattibo

I am trying to find the flow rate in a duct that is quite large (around 4'x3') but is very short. This duct leads to a fan which leads to a separate unit through another short duct section. Both these duct sections have different sizes (the fan outlet is much bigger than its inlet). I will take static pressure readings before and after the fan to calculate the flow rate. How will the change in duct sizing effect my flow rate estimation? I guess that the dynamic pressure head variation will have a significnt effect.

Thank you for reading.