Change app window size from manifest file

I want to test an app on a smallscreen device but all I have开发者_如何学Go is a Samsung Galaxy tab. So I thought adding

<supports-screens android:smallScreens="true"></supports-screens>

to the manifest will decrease the window size. But it doesnt work! I even tried setting support for large and normal screens to false but in vain. So I had to change the app's window size within the onCreate of the main activity. But I want to know why wont the above line in the manifest make any change to the window?

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If you are viewing the app on Tablet and willing to get the small screen size view, you may also need to add this attributes :

android:xlargeScreens="false"