While working, the font size has become too big by mistake and I am unable to reduce the font size. It is difficult to work with bigger font size. How to reduce it?

Let's get your QuickBooks Online (QBO) fonts changed, sathishkumar-s. 

To start, let's check the settings on your computer to make sure it displays all pages properly. QBO requires these settings:

• Screen resolution set to 1024x768 or higher
• Dots-per-inch (DPI) set to "Normal" or "100%"

Then, refer to the steps below to change the font size in your browser: 

1. On the upper right, go to the menu icon (looks like a three-dotted line).
2. Locate the Zoom option.
3. Choose the plus (+) icon to zoom in and make the page larger. It should be set to 100% or lower to display the full page.

Please take note that this will work on any page that's either too big or too small, not just QBO. We can also hold the "Ctrl" key and press the "+" sign simultaneously on your keyboard. Or, hold the "Ctrl" key and then scroll up. However, if you want to increase the font size on your printed sales forms, just edit it in the Custom Form Styles window. Here's how:

1. Go to the Gear icon at the top right.
2. Select Custom Form Styles.
3. Look for your form style, and then click Edit.
4. Go to the Design tab.
5. Click on the Select a different font option.
6. Select your desired font size. We have three choices available: 8, 10, and 12pts.
7. Click Preview or Print to see how your form would look like, and then click Done. 

To learn more about this one, see the Customise invoices, estimates, and sales receipts in QuickBooks Online article. Feel free to visit our Help Article page for more insights about managing your business.

Feel free to update us after performing the steps, as I want to ensure this is resolved for you. Please reply to this post and I'll get back to you. Take care always.