android:theme="@android:style/Theme.Dialog"

@Override
protected void onCreate(Bundle arg0) {
    super.onCreate(arg0);
    setContentView(R.layout.myxml);

    LayoutParams params = getWindow().getAttributes();
            params.height = LayoutParams.FILL_PARENT;
            params.width  = LayoutParams.FILL_PARENT;
           getWindow().setAttributes((android.view.WindowManager.LayoutParams) params);
   }

getWindow().setLayout(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT);

@Override
protected void onStart() {
   super.onStart();
   // In order to not be too narrow, set the window size based on the screen resolution:
   final int screen_width = getResources().getDisplayMetrics().widthPixels;
   final int new_window_width = screen_width * 90 / 100;
   LayoutParams layout = getWindow().getAttributes();
   layout.width = Math.max(layout.width, new_window_width);
   getWindow().setAttributes(layout);
}