stage.nativeWindow.width how to call

I have called this in a On a ENTER_FRAME event and it works but it causes my program to be buggy so I want call it only when someone resizes the window is there any way to do this

function sizewidth() {

if(stage.nativeWindow.width >800){

BIG()

}

if(stage.nativeWindow.width <800){

SMALL()

}

}

//addEventListener(Event.ENTER_FRAME, onEnter, false, 0, true);

function onEnter(evt:Event):void {

sizewidth()

}