Ans:

The measure() method sets the default component size, in pixels, and optionally sets the component's default minimum size.

Flex schedules a call to the measure() method when a call to the invalidateSize() method occurs. The measure() method executes during the next render event after a call to the invalidateSize() method. When you use the addChild() method to add a component to a container, Flex automatically calls the invalidateSize() method.

for example:

package myComponents
{
    import mx.controls.Button;

    public class BlueButton extends Button {

        public function BlueButton() {
            super();
        }

        override protected function measure():void {
            super.measure();

            measuredWidth=100;
            measuredMinWidth=50;
            measuredHeight=50;
            measuredMinHeight=25;
        }
    }
}

The following application uses this button in an application:

<?xml version="1.0"?> 
<!-- asAdvanced/ASAdvancedMainBlueButton.mxml -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
    xmlns:MyComp="myComponents.*" >

    <mx:VBox>
        <MyComp:BlueButton/>
        <mx:Button/>
    </mx:VBox>
</mx:Application>