As stated in my previous post I re-installed OpenSuse. In an earlier install a few months back I had the problem that my fonts were to big after installing the NVidia propietry driver.
Back then I had no clue what caused this problem. It appears that installing the propietry driver initiated the problem.
On a blog I thought I found a solution. But somehow it didn't resolve it for me.
What I also tried is to add windows fonts to my fonts system. Now I wanted those anyway since I have to exchange documents with colleagues that have windows. And for our company we have selected a company-style-font.

But what it I found that in my personal fonts settings the fonts DPI were not set correctly. Since I have a pretty high resolution laptop-screen (1920x1200) this is probably interpreted incorrectly. By default it is set to disabled, but you can set it 120 or 96. Setting it to 96 I got reasonably sized window-fonts.