The trouble with that is all lines have two ends, changing the length moves what where - do you want to hold the center and move both ends an equal amount, do you want to hold the 0 end and move the other, do you want to hold the max end and move the 0 end, do you want to differentially move the two ends? The Properties Pane does not really handle that type of selection and input.
You can use the Trim / Extend line command to Trim or Extend Lines and Arcs by a Distance and then pick the end that you want to move as that command has the User Interface to do exactly what you are asking for here.
Alan