If you used AutoCAD before using Revit you likely went through a bit of withdrawal from using inches as a base drawing unit. With Revit you had to get used to typing 0 space 6 for 0'-6" or 0 dash 6. Well in Revit MEP there is one instance at least where you get to enter relevant values when working through your design.
First if you are interested I posted a little video overview of this subject. Otherwise read on...
I am referring to the duct size parameters as the title suggests. In the Family Editor > Family Types dialog we can choose to Add Parameters to our family. Aside from providing a clever name we also can choose the Discipline > HVAC which in turn offers quite a few choices for "Type of Parameter".
When you choose Type of Parameter: Duct Size, Revit will require you to enter values in inches, which is easier since most duct sizes are referred to by their size in inches.
There's a catch though if you like to add parameters by using the Associate Family Parameter "sneaky button" in the properties of elements in the Family Editor.
This route precludes the use of the HVAC Discipline, Type of Parameter choices as it defaults to common and its subset of Types of Parameters. Not FAIR I say! I'm sure it is just an oversight!
To use it you need to start with Family Types > then Add Parameter. Then you can Associate the family parameter with the Element's parameter.