Good day, Julie. 

For now, there's no option to change the font size in QuickBooks Online. However, you can zoom in to make the page bigger. Just hold the "Ctrl" key and press the "+" sign simultaneously on your keyboard. You can also hold the "Ctrl" key and then scroll up.

Here's an article for more details: Unable to see the entire page in QuickBooks Online.

Also, please take note that you can customize the appearance of your sales receipt when printing them. Check out this guide for more details: Customize invoices, estimates, and sales receipts in QuickBooks Online.

The Community is always here if you need anything else.