If you don't have control of the html page where the ad is being hosted,
there will be a flash API supplied with whatever ad system is being used
to support such ads, so a generic approach won't be suitable in such cases.
Essentially you make a Flash design for the larger size, but mask it to
fit the smaller add display. When you wish to expand the ad you can
expand the mask to cover the larger area after calling the advertising
API to tell it to expand the HTML element that encloses the flash area.
I did this last year and it worked via a series of callbacks, but will
vary according to the ad system being used.
Paul