Hi Ol, so there is a plug-in that can help you with that, it's called UnwrapCurve and you can find it here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5136.1

It will create a line that is the length of the curve you have selected, and then if you select the line the properties panel in the upper right corner of the main window will display its length.

Or also there is a script here which you can set up on a shortcut key that will put the length of the selected curves as text in the clipboard that you can then paste in:
http://kyticka.webzdarma.cz/3d/moi/#CurveLength

- Michael