Yes, you can use the onClipEvent(data) event to catch the
moment when your swf was completely downloaded. And then you have
the _width and _height of that movieclip set to exactly what your
loaded swf sizes were.
Do something like this:
// for example if you need it to be always 800x600
var must_be_width = 800;
var mul_val = this._width/must_be_width;
this._width /= mul_val;
this._height /= mul_val;
// now your movie has the width 800 with proportional height
// no matter what the original size was.