I would just like to mention that I have set the body font to 25px to demonstrate the increased spacing between the lines and that the font in the blog overview stays as small as it is.
Unfortunately this does not solve the problem.
My problem is, that, no matter which font-size I define, it does not change the size of the font in the blog entries on the main page. Only the spacing between the lines becomes bigger.
There is no difference if I define the font-size in the theme settings or if I add it as a custom CSS.