How to change the labels font size
Yes there is a option to change the font size. The UI setting is not user friendly but it does allow you to update the attributes.


Navigate to CiviCRM >> Administer >> System Settings >> Option Groups.
Find for Mailing Label Format.
Click on "Edit options' link besides 'Mailing Label Format'
You will see list of formats, click on 'Edit' besides one of the format type.
In 'value' text field find for 'font-size' and change the value from old to new


for eg if the old value was "font-size":8 than replace it with "font-size":16

HTH

Pradeep