Thanks!
Edit: yes its CSS, also the meta data of maximum-scale=1.0 is already set in the index html, so it should have been applied in the first place. I hope the enforced 16 pixel will do the trick.
Old answer:
Since there is no such setting on the text input, I guess I have to do it with CSS? (Curiously, I can disable the setting of "auto font size", but doing so does not pop up a font size setting.)