Since you're shooting a rope, you have a start and an end point. Divide the length by the distance one rope texture "piece" should have and you get the scale factor.

Vector3 start = new Vector3(1, 2, 3); // just a random value
Vector3 end = new Vector3 (4, 5, 6);

float tileDistance = 2; // 2 world units per tile

void Update()
{
    float length = (start-end).magnitude;
    float numTiles = length/tileDistance;

    // scale the texture along the y axis
    rope.renderer.material.mainTextureScale = new Vector2 (1, numTiles);
}