Supported screen sizes are defined in AndroidManifest.xml by <supports-screens /> directive (placed directly in <manifest>> element).
It's unclear a bit which sizes are supported by default. It differs by API/Android version. So here comes my observation:
Android 1.6 (API 4): normal
Android 2.2 (API 8): small, normal, large
So, it's necessary to enable large and small screen support for Android 1.6 explicitly to prevent "compatibility" display mode of GUI.