The stage inherits whatever size you drag and expand it to when published. It doesn't really make sense to "set" it since you can resize it however you want externally. If it's in an HTML page, just set your HTML element/div width/height and the SWF will inherit whatever size the element/div ends up being. On the other hand, if it's a Flex/AIR app and is used in a standalone non-browser context, that's a little different:
[SWF(width='800',height='600',backgroundColor='#000000',frameRate='30')]
That's Flex-specific though and cannot be used in a regular Flash project.
If it's regular Flash though, it seems like you're thinking it out in the wrong way. Resize this page as an example - the stage resizes to the full browser width/height and the textfield just re-center's itself based on that size.