I believe you'd need something (code below untested) along the lines of...
var screen_aspect_ratio = Screen.Width / Screen.Height;
var outer_aspect_ratio = OuterMovieClip.Width / OuterMovieClip.Height;
var new_outer_width;
var new_outer_height;
if (screen_aspect_ratio > outer_aspect_ratio) {
new_outer_height = Screen.Height;
new_outer_width = (Screen.Height * OuterMovieClip.Width) / OuterMovieClip.Height;
} else {
new_outer_width = Screen.Width;
new_outer_height = (Screen.Width * OuterMovieClip.Height) / OuterMovieClip.Width;
}
OuterMovieClip.Width = new_outer_width;
OuterMovieClip.Height = new_outer_height;