How can i change the font size?

Hello there, @ACC-alarqam. 

Welcome to the QuickBooks Community! I'm here to help you change the font size in QBO. 

The font size in QBO depends on the Window settings. You can follow these steps if you want to change it. 

1. Select the menu icon (a three-dotted line) at the left section of the search bar.
2. Pick the Settings, then go to the Appearance section. 
3. Tick the Font size dropdown and make the necessary changes.
4. Refresh the page before re-opening your account.

To personalize your template, you can explore the "Custom form styles" through this guide:  Customise invoices, estimates, and sales receipts in QuickBooks Online.

If you want to know how to record invoice payments, you can visit this article for a step-by-step process: Record Invoice Payments In QuickBooks Online.

Let me know how this goes by commenting below. I'll be right here whenever you have follow-up questions about the display format in QBO. Have a great day!