Changing the default font settings for a workbook isn’t available in Excel Online. But you can change the font style and size for the entire worksheet.
If you have the Excel desktop application, and if you’re working with a blank Excel Online workbook, you can open it in the Excel desktop application and change the default font there. Here’s how:
Click Open in Excel and follow the steps in Change the font size.
When you save the workbook and reopen it in Excel Online, you'll see the changes you made.
For news about the latest Excel Online updates, visit the Microsoft Excel blog.