ActionScript 3.0 :: Width/height As Parameters To A Sprite Subclass

In a previous thread I asked a question about setting the width and height of a sprite subclass in its constructor method. One of the responses has puzzled me since I read it:
 
"Setting width and height only makes sense if you're adjusting the width and height of existing content, and the Chart (Sprite) has none at that point."
 
I don't understand that; what if you're adding children to the sprite that depend on those dimensions?
 
Here's a simple app which draws a red circle inside of a blue box which demonstrates my confusion as well as one other issue. The full questions are at the bottom.
 
Main.fla:
import Box;
var myBox:Box = new Box(200, 200);addChild(myBox);myBox.drawCircle();

[Code].....
 
Question 2: In this example, the dimensions of Circle are based on the width of its parent Box sprite. Is there a more elegant way to pass the dimensions to the circle? I ask because in my actual project (a chart) the plot area contains a lot of screen locations that must be passed around when I add new objects such as grid lines and markers and passing around the coordinates as parameters seems awfully messy. I'm just not sure how else to do it.