I hope your day is going great. Like my colleague ReymondO said, able to change the font size on the report is unavailable. At this time, we can only adjust the screen size in QuickBooks Online (QBO) by manually holding the Ctrl key and press the Plus sign (+) or Minus sign (-) simultaneously on your keyboard.
I know how the feature you want would be helpful in the program. The Community recognizes feedback and suggestions from customers like you can help us improve our product and services to our customers' delight.
While we continue to make your experience with us even better, I recommend visiting our QuickBooks Blog to get the recent news, information about the latest features, and updates.
On the other hand, in case you want to change the font size on your printed sales forms, you can do so by modifying it in the Custom Form Styles page. I'll show you how:
Go to the Gear icon and select Custom Form Styles.
Look for your form style, and then click Edit.
Go to the Design tab and click on the Select a different font option.
Select your desired font size. We have three choices available: 8, 10, and 12pts.
Click Preview or Print to see how your form would look like, and then click Done.