Well there is no set size you can use to make everything look pretty, sadly. You will notice most sites use pop ups with size specific flash. This way you can control the browser size with the pop up.
If this isn't an option, the recent trend has been setting your flash dimensions to 100%x100% and using the Resize listener to detect when the user changes the browser size. Then you will wanna set your flash Scale mode to "No Scale or "Exact Fit". Depending on your needs. This will allow you to scale your flash to fit the browser. Instead of trying to make the browser fit your content.
Check out
http://www.gotoandlearn.com/
and select the "Full Browser Flash" Video tutorial, that should help you out.
also the option of using the new FullScreen mode in the flash 9 player, example here
http://www.bezzmedia.com/swfspot/tutorials/intermediate/True_Fullscreen_Fla...
hope that helps ya out.
-ross
