Fonts. You can adjust invoice appearance by change fonts family and fonts sizes. Note: This options affects also on the Statement as well as the Packing Slip.
Items table. For better appearance and readability you can show or hide horizontal and vertical lines in items table. You can use different combinations and choose appropriate for you. Moreover if you have only few items on invoice, for better look check on 'Show empty rows'. This option adding some empty rows what makes the table is bigger.