I'm a bit confused and don't follow your description. It's not exactly a math problem, but still... The issue with a spline is that there is no mathematical formula to give its properties like length, width, etc.
Is the spline 3D? Can you trace a polyline or linestring over it and have the software measure that instead? It's a graphical as opposed to mathematical solution and can be time consuming, but it gets you there in the end.