The standard labels do not have an option to change the font size, but you do have the ability to change the font in the Custom labels. Once you go to Students>Print Avery Labels and select your students through the search, choose the radio button for Custom Label and click Edit, as shown below:
A pop up window will appear where you can select your font size as well as the fields you want in your label.
Click Continue when you are finished selecting your fields, check the boxes to the left of the students that you want to print labels for, and click Create Labels for Selected Students in the upper right hand corner.
Once you have previewed and printed your labels, click on Return to Focus in the upper Left Hand corner, and you will see that the criteria you used for your custom label will be saved for future use.