I've found a link to a thread at Blenderartitsts.org that discusses this problem and possible solutions:

http://blenderartists.org/forum/showthread.php?206790-Length-of-a-Bezier-curve-in-2-5&highlight=bezier+curve+length

On the final page someone has made an addon to do what you ask. I would suggest reading through to see whether it fits your needs. If you just want to jump in I'll attach the addon: Remove the .txt extension before use.