To change the size of the page number on the pages of your bundle, first navigate to the index and pagination settings by clicking the "Index and Pagination" row on the bundle building page.
This will reveal a side panel on the right containing the index and pagination options. Under the "Pagination Settings" section you can use the "Size" dropdown to change the pagination font size.
If you've already generated your bundle before changing these settings, please regenerate your bundle again for the changes to take effect.