I have never seen a option to choose font size in the menus of any dashcam, do not have knowlege to guess if it was possibel to put that in as a option.
Like my Lukas i prefer all info to be along the bottom of the footage, witch in most cases are dashbord or front of the car anyway.
When you export a clip you can add/change the font via http://registratorviewer.com
While this doesn't effect the existing hard coded font you add an additional one.