Hi Uwe,

> | #+HTML_HEAD_EXTRA: <style>*{font-size: 108%;}</style>
>
> Now some text is changed as expected the text in the header not really.
>
> Any idea what would be a more appropriate setting?

Try changing the body font size. This /should/ change everything. If not the
default CSS should be adjusted (let me know and I will do so).

All the best,
Timothy