Change TextSize of TabbarTitle in Android Programmatically

I know there are lot's of samples of doing this but none of them works for me. I can change the colors, even the Text of a Tabbar Title but not the the size nor font. I tried through Custom TabbedPageRenderer. And I have a simple Demo project with a ExtendedTabbedPageRenderer.cs in the Android part.
The Application class is a FormsAppCompatActivity.