Also, If I put a value to the "Takeoff Fixed Length", It does not change. I am seeing the parameter formula, and it should override the "Takeoff Length". It is not doing so.
I took a look at the family Round Takeoff.rfa, in the US Imperial\Duct\Fittings\Round\Taps folder, and found three parameters for length:
Takeoff Length Projection
Takeoff Fixed Length
The first two parameters can be manually adjusted, but the third has the following formula:
if(Duct Diameter > 1' 4", Duct Diameter * 0.25, 0' 4")
I am not clear which length you are referring to, when you say "What determines this length".
If you are referring to the "Takeoff Length" parameter, then this value is specified in family, and you can setup separate values in different Family Types.
Note: I placed several instances of the Round Takeoff family into a new project with different types specified with different duct diameter, and I found the width of the element was not changed (it was always equal to the sum of the Takeoff Length Projection and the Takeoff Length parameters).
Funny, I don't have any formula for Takeoff Fixed Length...
The only thing that "Manually" Changes the length is the Takeoff Length. I see a formula under "Length", but even this does nothing to control the actual Length of the Tap. If I do not override the "Takeoff Length", the tap gets longer when the duct radius gets larger, however there is no formula controling this. It is probably internal, based on an ashrae table, I just wanted verification that that was true.
It's just funny there is 4 parameters for length but none does anything to control it.
It sounds like you are looking at a different family.
What is the name of the family file that you are working with (and did it come with the imperial library, or did you download if from somewhere else)? Also what version of Revit is it coming from?
The family is "Round Duct Takeoff". Now that you mention it, i looked in the 2012 OOTB families, I guess it is different. The 2012 "Round Takeoff" has the same parameters, but doesn't change size at all. it remains 4" long no matter what diameter the duct is. I guess you are supposed to manually go in and apply the "Takeoff Fixed Length" parameter to the "Takeoff Length"?