Button and size of text

I have a problem with size of text in button. I have layout where I use weightSum and my button width is match parent and hight is 30%. Everything works fine but when I change size of screen on emulator my text is still small. How I can change size text depend on size of screen?
This is xml:

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:orientation="vertical"
     android:weightSum="100" >
 
     <Button
         android:id="@+id/button1"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:text="Button"
         android:layout_weight="30" />
 
 </LinearLayout>
 

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