In Writer I can establish a database of addresses. I can print labels. What I cannot do is change the font size on the labels. At what step in the process is font size specification accomplished? How?
I can’t tell you about labels merged from a database, but in general select SYNCHRONIZE CONTENTS on the options tab of the Labels Dialog so that any font changes can be applied to all labels. Click NEW DOCUMENT which will show the page(s) of labels. You can select all (Ctrl-A) and then modify fonts; clicking SYNCHRONIZE LABELS will apply the change to all labels.
I have limited experience with mailing labels / mail merge. What I found is that once the field names are placed in the Writer document, they can be selected and their size set. That size then carries over to the labels that result from executing the merge.