in my experience, unfortunately Flash will only return the size of the window (stage width & height) if allowScale is turned off, which means you have to first switch it off, get a size, then scale your stuff repositioning 0,0 etc manually afterwards.

Regrettably this results in you almost building your own liquid layout browser rendering engine - especially if aspect ratios are different too!

Flash & Flash Lite's Fullscreen command is brilliant, and coupled with the scaling of vector graphics it makes scaling easy, so things look better if you avoid bitmaps in your app elements as much as you can - but the above issue is a pain in the butt.

I have this problem too with an app originally designed for 240x320, now of course it has to run on 480x800 (2 years later)

Have a look through the other posts in the board outside the mobile forum, I remember posting something elsewhere about this that might help you.