I don't know what a curved line is, but "_list" is right.
Or use the command "_lengthen"
Or look in the properties window (Since AC2000)
Or...to be continued...
On my machine, Exxit's first 2 suggestions also give the length of a spline, "list" and "lengthen". The Lengthn command doesn't work on the spline but it does give the length.