50 MB is the current maximum, however keep in mind that's for the entire app, including all activities and things like icons and background images.
Also: for apps above a certain threshold (I think around 25 MB but it may be smaller) the Android Market app will show a "WiFi" tag, and the app will need to be downloaded over a wireless network to avoid data charges.
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Also: for apps above a certain threshold (I think around 25 MB but it may be smaller) the Android Market app will show a "WiFi" tag, and the app will need to be downloaded over a wireless network to avoid data charges.