If your motion of the sword is based on the mouse movement, you could actually use the mouse delta values as your "damage amount" value.

Another method would be to create a dummy gameobject which is parented to the tip of the sword, and measure its position each frame. You can then calculate its speed by comparing the difference between the last measured position and the current position.

Eg:

var swordTip : Transform;  // drag a reference to the swordtip dummy object here
private var lastTipPosition : Vector3;

function Start() {

    lastTipPosition = swordTip.position;

}

function Update() {

    var swordSpeed = (lastTipPosition - swordTip.position).magnitude;
    lastTipPosition = swordTip.position;

}