But maybe you meant something else? The width and height of an swf are coded inside it and could be changed with some editor, like a hex editor. But then you would have to know where the bytes are that you need to change.
Do you need to alter its dimensions or its length? If it's the dimensions you want to change then it could be as easy as changing the width and height parameters of your embed code, or loading the swf into another one and displaying it behind a mask that shows only the parts you want to show. No change necessary inside the actual swf.