SWC LivePreviewParent'width can't be changed !?

Here is a picture:

The black border is SWC component's bounds.The blue one is what you select an element on stage will show.I want to change the blue one in my component's code.But I found the blue one is a LivePreviewParent object. Changing width is change the component's width in it. But I just want to change the LivePreviewParent itselft, like typing width number on Flash porperty panel. How can I do ?