[dee2]

Stage.height

Description

Property (read-only); indicates the current height, in pixels, of the Flash movie Stage. When the Stage.noScale property has a value of true, height represents the height of the Flash Player. When the Stage.noScale value is false (movie scales when player window resized), height represents the height of the Flash movie.

(с) Macromedia Flash MX help

Stage.width - аналогично

if(this.hitTest(x,y,true)){
if (_root.mc._y<Stage.height ){
_root.mc._y = _root.mc._y +2;
}}

только тестируй отдельно запущеный swf
Test movie дает другой размер stage