There are a few things to keep in mind when working with curves.
You can apply a weight map on a curve, but you cannot display it or paint on it. You can only use the constant or gradient base map generated by the WeightMapOp operator.
Displaying Information on Curves
Press Shift+Enter or choose Edit > Info Selection to get useful information about the selected curve object, including its length in Softimage units.
Autodesk Softimage v.7.5