If it is to be a utility app, where your code is drawing all of the interface, you could take on the size of the screen you are given, and lay out the buttons and fields to fill the area.
If the app is more like animation, ActionScript has different stage modes you can use. The default one is ShowAll. With ShowAll the stage is scaled so that all of the stage content is visible, and if the device screen is wider than the scaled stage width, whatever content was off the sides of the stage will be revealed. If the device screen is taller than the scaled stage height, whatever was off the top and bottom of the stage is revealed.
So, you would then design the animation to fill a stage that is the midpoint of what you think devices will have available. Screen rations are roughtly (these are landscape numbers) 2:1 to 4:3. If you designed your animation on a 16:9 screen, with extra graphics above and below that, 4:3 screens would be shown the extra vertical content. If you're on a 2:1 wide device, the extra content to the left and right of the 16:9 stage would be revealed.
As vectors scale niceley, and with bitmaps you could have them at say 50% if their true size, using a stage of 1920x1080 would still be good looking on a much larger device. To have enough content for a 2:1 device, you would want graphics as wide as 2160, and for 4:3 devices you would want graphics as tall as 1440. You would make sure everything that is important is visible in the 1920x1080 area, and have less important graphics in the areas outside of that.
To play safe, for devices that are wider than 2:1, you may as well have graphics as wide as 2560. That would be enough for devices that are 2.37:1 wide.
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