Screen sizes beyond phone screens was added in Donut (API 4). Thus to say that you know about different screen sizes, you need to set at least API 4 as the target SDK version in your manifest:
<manifest ...>
<uses-sdk android:targetSdkVersion="4" />
</manifest>
This allows the system to use the modern defaults for the supported screens that you can specify with this:
http://developer.android.com/guide/topics/manifest/supports-screens-element.html
The use of android:targetSdkVersion is very important, since it allows the platform to disable a variety of compatibility behavior for your app to allow it to run better on more recent versions of the platform. Some of the things it impacts can be found here:
http://developer.android.com/reference/android/os/Build.VERSION_CODES.html