To change the size of the font 2 steps need to be done:

1. Add as Style for the Tablayout
2. Apply the style to the TabLayout in the XML

Find or create res -> values -> styles.xml

1. Add the Style to the XML

<style name="MyCustomTabText" parent="TextAppearance.Design.Tab">
    <item name="android:textSize">12sp</item>
</style>

2. Apply Style

Find the Layout containing the TabLayout and add the style. The added line is bold.

<android.support.design.widget.TabLayout
    android:id="@+id/tabs"
    app:tabTextAppearance="@style/MyCustomTabText" 
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

Add this to your mainactivity:

@Override
public boolean onCreateOptionsMenu(Menu menu){
    // Create menu
    // See onOptionItemSelected for actions when clicked

    Menu settingsMenu = menu;
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.optionmenu, menu);

    MenuItem settingsMenuLogin = settingsMenu.findItem(R.id.settingsMenuExport);

return true;
}

and

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();


    if (id == R.id.settingsMenuExport) {
        // add additional check
        Intent i = new Intent(getApplicationContext(), export.class);
        startActivity(i);
    }
    return super.onOptionsItemSelected(item);
}

and create an xml in res\menu like this:

optionmenu.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/settingsMenuExport"
        android:orderInCategory="100"
        android:title="Export"
        app:showAsAction="never" />
</menu>