[Hauts]

_diam = _radius / 2;

Диаметр = радиус делить на два?

Добавлено через 7 минут
Main.as:

package 
{
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
 
	/**
	 * ...
	 * @author Hauts
	 */
	public class Main extends Sprite 
	{
		private var _ball:Ball;
		public function Main() 
		{
			addEventListener(Event.ADDED_TO_STAGE, onAdded);
		}
 
		private function onAdded(e:Event):void 
		{
			removeEventListener(Event.ADDED_TO_STAGE, onAdded);
			_ball = new Ball();
			_ball.x = _ball.y = 100
			addChild(_ball)
 
			stage.addEventListener(MouseEvent.MOUSE_MOVE, mouseMoveHandler);
 
			mouseMoveHandler(null)
		}
 
		private function mouseMoveHandler(e:MouseEvent):void 
		{
			_ball.diameter = (stage.mouseX - _ball.x) / 2
		}
 
	}
 
}

Ball.as:

package  
{
	import flash.display.Graphics;
	import flash.display.Shape;
	import flash.display.Sprite;
 
	/**
	 * ...
	 * @author Hauts
	 */
	public class Ball extends Sprite 
	{
		private var _shape:Shape;
		private var _radius:Number;
		private var _diameter:Number
		public function Ball(diameter:Number = 10) 
		{
			super();
 
 
			_shape = new Shape()
			addChild(_shape)
 
			diameter = diameter
		}
 
		private function render():void 
		{
			var gr:Graphics = _shape.graphics;
			gr.clear();
			gr.lineStyle(1)
			gr.beginFill(0xFF0000);
			gr.drawCircle(0, 0, _radius);
			gr.endFill()
		}
 
		public function set radius(value:Number):void {
			_radius = value;
			_diameter = _radius * 2;
			render();
		}
		public function set diameter(value:Number):void {
			_diameter = value;
			_radius = value / 2
			render();
		}
		public function get radius():Number {
			return _radius
		}
		public function get diameter():Number {
			return _diameter
		}	
	}
 
}