Stage height and width in private const or variable

Question

Hello, everyone I've got problem with this code.
I need stage size in const or variable because of some resize reasons.
ok so, code below works but it's static and I need dynamicaly get stage size.
private const LIST_HEIGHT          :uint = 800;
private const LIST_WIDTH            :uint = 1200;
with(_mask)
graphics.beginFill(0);
graphics.drawrect(0, 0, LIST_WIDTH, LIST_HEIGHT);
graphics.endFill();
and if I change constants to this:
private const LIST_HEIGHT          :uint = stage.stageHeight;
private const LIST_WIDTH           :uint = stage.stageWidth;
it's not work
also this not work
private var _listH          :uint = stage.stageHeight;
private var _listW         :uint = stage.stageWidth;
and this:
private var _listH          :String = stage.stageHeight;
private var _listW         :String = stage.stageWidth;
not work means that I get this error
TypeError: Error #1009: Cannot access a property or method of a null object reference.
          at com.filebrowser::FilesList()[/Users/Peter/Desktop/projectfl/projectfl beta 0.6/as3/com/filebrowser/FilesList.as:14]
          at flash.display::Sprite/constructChildren()
          at flash.display::Sprite()
          at flash.display::MovieClip()
          at com.filebrowser::FileBrowser()[/Users/Peter/Desktop/projectfl/projectfl beta 0.6/as3/com/filebrowser/FileBrowser.as:51]
Thanks in advance for all replies, all help is very appreciated.

Answer 1

ps. I should add this information.
this is code for external swf.
if it's alone it works but if I load it to main swf it's not working.
and my only problem is this:
this is working
private const STAGE_HEIGHT:uint = 800;
and this not
private const STAGE_HEIGHT:uint = stage.stageHeight;