How to make setting to change all text in app font size from preference

Question

How do I make a setting to change the font size of all text in my app from preferences.

My preferences xml is

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
<CheckBoxPreference
    android:defaultValue="true"
    android:key="check box"
    android:summary="Screen stays on yes/no "
    android:title="Display stays on"
    />
<ListPreference
    android:title="Font Size"
    android:key="Fontsize"
    android:entries="@array/listarray"
    android:entryValues="@array/listvalues"
    android:defaultValue="3"
    />

and arrays

<string-array name="listarray">
    <item>Very Small</item>
    <item>Small</item>
    <item>Medium</item>
    <item>Large</item>
    <item>XLarge</item>

</string-array>
<string-array name="listvalues">
    <item>1</item>
    <item>2</item>
    <item>3</item>
    <item>4</item>
    <item>5</item>


</string-array>

and preference.java

public class preference extends PreferenceActivity {
@Override
protected void onCreate(final Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getFragmentManager().beginTransaction().replace(android.R.id.content, new preferencefrag()).commit();
}





public static class preferencefrag extends PreferenceFragment {
    @Override
    public void onCreate(final Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        addPreferencesFromResource(R.xml.preference);

I want to change size of all text in the layouts off the app by the list preference