Your question is rather vague, but I will try to answer it.
Let us say you have a sketch with a bunch of fully defined (by dimensions) dots. You then connect the dots. If you connect the last two dots (to create a closed area), you can use the "Area" command under the Inspect menu to create an area. After that, you can use Goal Seek with the perimeter as your set variable (you will need to substract the length of the last segment you put in to complete your area) and a point dimension as your varying variable. Set the perimeter (or total length) and watch as SE finds you a solution.
The length of a b-spline is also part of it's properties (while in sketch mode, right click on b-spline, properties; there should be a box labeled length), and if it's possible to make these properties variables, you would be able to set them manually. I'm very doubtful of this though.
The curve length is determined by the points you've defined and it's end conditions and can't be made a driving dimension.
You can't even make the length measurement a variable, as you can with a distance measurement between faces etc.
