Hi nos, see here for a shortcut that will copy the length as a number to the clipboard just like you were describing:
http://kyticka.webzdarma.cz/3d/moi/#CurveLength

But if you want to create a line that's the same length as the curve, you may want to get the "UnwrapCurve" plugin which automates the creation of such lines, see here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5136.1

- Michael