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?

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"