Question

The usable stage size limit in Flash Player 10 is roughly 4,050 pixels by 4,050 pixels. However, the usable size of the stage varies depending on the settings of the QUALITY tag. In some cases, it’s possible to see graphic artifacts when stage size approaches the 3840 pixel range.
http://kb2.adobe.com/cps/496/cpsid_49662.html

But what about Flash Player 11?

Answer 1

Standard marketing sights: Enhanced high resolution bitmap support

http://www.adobe.com/products/flashplayer/features.html
http://www.techspot.com/downloads/1127-adobe-flash-player.html

Enhanced high resolution bitmap support — BitmapData objects are no longer limited to a maximum resolution of 16 megapixels (16,777,215
pixels), and maximum bitmap width/height is no longer limited to 8,191
pixels, enabling the development of apps that utilize very large
bitmaps.

O’Reilly Media cites:

1 megapixel is equal to 1,000,000 pixels.
Flash Player 10 supports up to 00.008 megapixels.
Flash Player 11 supports up to 16.777 megapixels.

Although, the Adobe Flash Player 11 / Features page lists previous constraints for effects.

http://www.adobe.com/products/flashplayer/features.html

3D effects Manipulate large bitmaps up to 16,777,216 pixels
(4096×4096) with a maximum length of 8191 pixels per side.

Posts regarding this topic at the Adobe Forums seem unresolved, such as:

Maximum bitmap width/height in Incubator?
http://forums.adobe.com/message/3898280