You can manage your check-in labels by clicking Settings and when the page loads, clicking on Check-in. Once you’re there, look for a heading called ‘Label Designs’.
Under this area, you can adjust your font size by using this shortcode around the label fields.
[size=13]Text goes here[/size]
Bear in mind the font size is relative to the amount of text on the label. If the label has a small amount of text, then the font size will appear as larger. However, if there is a large amount of text on the label, then the font size will appear as smaller. This is done to help ensure text always fits on the label.