Sounds fun.
Heres the deal (apologies if you already know this) - expandable Flash banners are not 1 file, but 2. The flash banner file does not actually grow, but is replaced by the second bigger Flash banner. The code to make the html expansion is typically provided by the ad vendor as well as the Flash template files. (Flash talks to the on page script)
Do you have the vendor specs? or is this a file you will be hosting on your own site?
I am asking because I will need to know if I am only animating your designs or building code to make the expansion happen as well.
Look forward to working with you.
Regards,
Jeremy