Hi propmaster, there is a plug-in here that will show the length of the current curve that is being drawn:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2703.12

If you want to enter in a length you could do that by formula. Check out here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2558.2

- Michael