How do i enlarge the text on the"message on invoice" ?

I'll show a way where you can adjust the font, pam@lloydangove.

You can modify the invoice template by following these steps:

1. Go to the Gear > Custom Form Styles.
2. Select the Edit link of the default invoice template.
3. In the Design tab, click the Select a different font section and change the font size.
4. Hit Done.

Here's a sample screenshot for a visual guide: 

You can also read this article for more customisation: Customise invoices, quotes, and sales receipts in QuickBooks Online.

I'll be around if there's anything else you need help with QuickBooks Online. Thanks.