MovieClip proportions including mask

I’m just starting out with AS3 and I’m trying to adjust the width and height of a MovieClip that contains images that are masked. For instance the main container clip is called MainClip and inside MainClip I have a large image at (1000x1000) masked down to a 500x500 square. If I trace MainClip.width I will get the overall size of the clip up to the image’s original size which is 1000x1000 (even though it is masked). How do I access the overall clip’s size including the mask? I realize I can access the mask width, but I’m actually trying to SET the width of MainClip to a larger size and when I do so it doesn’t do it according to it’s visible mask but via it’s children. Thanks for the help.