depenfing on how You have defined this cable or that part You will find several options to fix the length of it.
If this is an ASM and a cable/wire harness or a pipe (xPresRoute) then You will have a path option for those splines to set a fix length.
If it is a part with a spline sketch You simply need to deimension it using a smart dimension.
This will define the spline length
Thanks for getting back to me, i tried the smart dimension and it only gave me the radius of the sections.
im using st4. Is the length option something introduced in later versions? (Drawn in a sketch).