How to change the toolbar text size?

You can add a TextView to your toolbar and customize as you want:

<android.support.v7.widget.Toolbar
    android:id="@+id/toolbar"
    android:layout_height="wrap_content"
    android:layout_width="match_parent">
    <TextView
        android:id="@+id/toolbar_title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/app_name"
        android:textSize="25sp"/>
</android.support.v7.widget.Toolbar>

Use app:titleTextAppearance:

<android.support.v7.widget.Toolbar
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="?actionBarSize"
    android:id="@+id/toolbar"
    app:titleTextAppearance="@style/Toolbar.TitleText" />

and override the default title size in a custom style:

<style name="Toolbar.TitleText" parent="TextAppearance.Widget.AppCompat.Toolbar.Title">
    <item name="android:textSize">18sp</item>
</style>

Result:

for example this your toolbar

<android.support.v7.widget.Toolbar
    android:id="@+id/toolbar"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    android:gravity="center"
    android:minHeight="?attr/actionBarSize"
    app:theme="@style/ThemeOverlay.AppCompat.ActionBar" />

you can simple add this view this

    app:titleTextAppearance="@style/yourstyle"

style.xml

  <style name="yourstyle" parent="@style/Base.TextAppearance.AppCompat.Title">
    <item name="android:textSize">20sp</item>
</style>

like that...