Right now, you're not doing anything to the radius that depends on the length of the curve. You are setting the radius to, well, the existing radius, which doesn't do anything. And while you are displacing the eventual mesh object along its normals, you're not providing a vector the Voronoi texture, so you're relying on any defaults for this vecotr-- I'm not sure if any exist, but if they do, they're not based on curve length, and Voronoi doesn't trail off to zero anyways.
If we want to drive the radius of a curve in GN, it's fairly straightforward:
We use a spline parameter node to drive a function (here, a float curve node) which drives the radius. Shown with two splines within a single curve, to demonstrate that the length is per-spline, and here, using the "factor" output of spline parameter, that length is relative to the total length of the spline.
What if we want to instead drive the radius of the spline from length in absolute terms, rather than relative to the length of the spline? Then we can make radius some function of spline parameter/length, which outputs the object space length of the spline at each control:
These are the same input splines, but now they are mapped, via a map range node, to a radius of 1.0 at the beginning of each spline and 0.0 at 2 units along each spline. The top spline is still longer, but beyond 2 units length, it has a radius of 0, so they appear the same length. More precisely, every control that is more than 2 units along the length has a radius set to 0-- between controls, the radius interpolates. So this doesn't exactly mean that the visible spline is 2 units long.
If we would like to displace the radius after the curve has been turned into a mesh, perhaps to displace it in any direction, we can capture a spline parameter and use it to modulate that displacement:
Here, I'm scaling my mesh displacement down by the spline factor, so there is no displacement at the first control of the spline, and there is full displacement at the end of the spline.
We could use length instead for this, just as we did above; we could use a float curve or other function to modulate it, just as we did above.