Note: You cannot change the default font for an entire workbook in Excel Online, but you can change the font style and size for a worksheet.
When you enter data in a worksheet, Excel automatically uses the Calibri font style in size11 but if you prefer a different font style or size, here’s how you can change them:
Select the cell or cell range that has the text or number you want to format.
Click the arrow next to Font and pick another font.
To change font size, click the arrow next to the default Font Size and pick the size you want.
In our example below, we’ve changed the font style to Century and size to 9.
Tip: Data added to cells outside the newly formatted cell range would appear in the default style and size and you’ll need to repeat the above steps to format them. You can also select blank cells and apply the formatting before entering data.
You can also change the way the text is placed in a cell.