Adjust Transition Length
To adjust the length of a transition fitting, go to
Ribbon: 
Adjust Transition Length
Pulldown Menu:
You will be prompted to specify a new end point for a transition on the drawing.
Select duct or label at end of transition:
The transition will extend from the duct intersection toward the next duct. See the Previous and Next Ducts section for information about determining which duct is the next duct.
The end of the transition will be adjusted to the specified location.
Related Options
Angle of transitions: Sets the default angle of duct transitions.