Note! Please remember that the following changes are visible only on the printout or PDF generated from the system 

To change some element of the invoice on the printout, you need to use the appropriate CSS code in SETTINGS > ACCOUNT SETTINGS > PRINT SETTINGS (tab), and then fill in the Customize print with CSS field at the bottom of the page.

To make the text even bigger, you need to enter higher values, and to make it smaller, lower values.

.invoice_outline { font-size: 1.1em !important; } 

You can similarly change the size of other fields on the invoice.