I understand how frustrating it can be when the font size in a pivot table changes upon refresh. Here are some steps you can try to fix this issue:

1. Select the entire pivot table by clicking on the top left corner of the table (where the row and column headers intersect).
2. Right-click on the selected area and choose "Format Cells" from the context menu.
3. In the Format Cells dialog box, select the "Font" tab.
4. Set the font size to 12 and click "OK" to close the dialog box.
5. Refresh the pivot table and see if the font size stays at 12.

If the above steps don't work, you can try the following:

1. Select the entire pivot table again.
2. Right-click on the selected area and choose "Table Options" from the context menu.
3. In the Table Options dialog box, uncheck the "AutoFormat Table" option.
4. Click "OK" to close the dialog box.
5. Refresh the pivot table and see if the font size stays at 12.

If neither of these solutions work, you can try creating a new pivot table and formatting it with the desired font size before adding any data to it. This should ensure that the font size stays consistent upon refresh.

I hope these steps help you resolve the font size issue in your pivot table. Let me know if you have any other questions or concerns.