QuickBooks Online is a user-friendly way to make the most of your business bookkeeping. The cloud-based program helps make your bookkeeping flexible and efficient. While there's no way to adjust the font size within the QBO settings, I'd be happy to suggest a workaround to enlarge your screen in your web browser.
You're able to enlarge the magnification on your screen in whichever browser you may be using by clicking "Ctrl +" on your keyboard. This will magnify your browser screen. You can also revert this by clicking "Ctrl -".
I know this may not be exactly the solution you were hoping for, and I think having the option to control the font size within your QBO account would be a great addition to the product. I'd recommend submitting your feedback about this to our product development team by clicking the Gear icon > Feedback. I'll pass this along to my team on your behalf as well.
If you have any other questions, don't hesitate to reach out to our tech support team.