you can't change the font size except text/picture object.
QlikSense operates with responsive design, it tries to display the charts/texts/labels at the most optimal.
You can try use sub-legend instead of legend if you have a problem with size of legend.
Other tipp: change your browser resolution, and/or make it full screen and see the result in you dashboard (with qliksense desktop navigate to http://localhost:4848/hub/my/work then do the zoom in/out).
For the free Desktop product it's currently quite hard.
However if you are running Qlik Sense Server you can always change the theme which would allow you to configure font sizes for titles, sub-titles and foot notes.
Until we ship the full theming capabilities you can only have one theme that spans all apps.
You will find the theme.css file at C:\Program Files\Qlik\Sense\Client\themes\sense (assuming you installed on the C drive)
You can change font size, style, weight, well... anything you can do with css basically