I did some quick test. And usually, 3d Lenth parameter returns nothing. Actually, it seems like you can't schedule length of any MEP elements by standard parameters. I have tried to convert it to shape but you don't get any useful values.
What you can do is to chosen MEP_Straigth_Length parameter form duct objects and list those. However, it returns length only for straight ducts. I couldn't find any relevant parameter in the elastic duct. Probably there is the possibility to add some parameters calculating length based on the spline of the channel, but you will need someone who is good with GDL's.
