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For now, the maximum font size in QuickBooks Online (QBO) is up to 12 points (pt). You'll want to import your own custom form style so you can design your own invoice.
To give you more information and detailed instruction, you can read this article: Import Custom Form Style for Invoices.
You can also visit this article for a step by step process when receiving an invoice payment: Record Invoice Payments In QuickBooks Online.
Please get back to us if you have further questions. I'll always be right here to help you.