Stage height and width in private const or variable

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.