Ahhh well elsewhere on this website are family editor guides for both Revit Architecture and Revit Structural. I think the Structural one is from 2010 but looks and acts pretty much like the latest. I downloaded both to my PC for reference.
If you imagine working in good old orthographic projection (top, front and right side), you would define a reference plane in the right side view (also, name it). Next you dimension it. Next you edit the dimension to give it a label. In the label drop down you define a new parameter. This will be for length. When you define the parameter, click the radio button for Instance Parameter. Make sure the dimension is locked to the reference plane. Then make sure the extrusion length is defined as being to that parameter and not a fixed length. Next is to test it by gong into the family properties and changing the value of that length, and seeing if the extrusion adjusts to the new length (if not, then there is something not tied together like it should be). The reference plane should "flex" with the change in the parameter, and the extrusion should do so too.
If the paramater was made an instance parameter, then when you insert the family into a project, it should come in at the default length. But if you select one, then you should be able to overwrite the length in the properties browser, and it will change length. You can have many instances all at different lengths in your project this way. I hope that helped 🙂
If by some odd chance my nattering was useful -- that's great, glad to help. But if it actually solved your issue, then please mark my solution as accepted 🙂
