how to re-size a sphere using distance?

I have made a turret which can only shoot the enemy within certain firing distance.
Now, I want to indicate the distance using a sphere with transparent color.
However, I don't know how to create a sphere with a radius. I can't find any function in Unity can do this.:-|
Anyone done this before??

 

http://docs.unity3d.com/Documentation/ScriptReference/GameObject.CreatePrimitive.html

(then scale its size)

 

I know how to change the position but the main problem is how to set the scale to fit the distance.
Currently, the only way is to find the scale manually by changing the scale and comparing with the distance during the run-time.

 

Would this work?

var ourLocation : Vector3;
var theirLocation : Vector3;
var scaleAmount : float;

function Start(){

theirLocation = GameObject.Find("Main Camera").transform.position;

}
function Update(){
ourLocation = transform.position;
scaleAmount = Vector3.Distance(ourLocation,theirLocation);
transform.localScale = Vector3(scaleAmount/10,scaleAmount/10, scaleAmount/10);
}