how to set width which is equal to another widget on android

If you use a RelativeLayout you can use the align-attributes:

<View
    android:id="@+id/separator"
    android:layout_width="0dp"
    android:layout_height="0.3dp"
    android:layout_below="@+id/textView1"
    android:layout_alignLeft="@+id/textView1"
    android:layout_alignRight="@+id/textView1"
    android:background="#ffffff" />