The short one…
$ grep ActionBar platforms/android-11/data/res/values/*
leads to
styles.xml:
<style name="TextAppearance.Widget.ActionBar.Title"
parent="@android:style/TextAppearance.Medium">
</style>
<style name="TextAppearance.Widget.ActionBar.Subtitle"
parent="@android:style/TextAppearance.Small">
</style>
[…]
<style name="TextAppearance.Medium">
<item name="android:textSize">18sp</item>
</style>
<style name="TextAppearance.Small">
<item name="android:textSize">14sp</item>
<item name="android:textColor">?textColorSecondary</item>
</style>