trovato!

l'swf lo faccio al 100%

<td>
<object type="application/x-shockwave-flash" data="sfondo.swf" width="100%" height="100%">
<param name="movie" value="sfondo.swf" />
</object>
</td>

mentre nel clip:

onClipEvent (load) {
valX = System.capabilities.screenResolutionX;
valY = System.capabilities.screenResolutionY;
this._width = valX;
this._height = valY;
}