Here's how to make a cell's font size automatically adjust to fit the text:

1. Select the cell(s) you want to format.
2. Click on the "Home" tab in the ribbon.
3. Click on the "Alignment" group.
4. Click on the "Wrap Text" button to enable text wrapping.
5. Click on the "Format" button in the "Alignment" group.
6. Click on the "Autofit Row Height" option.

This will automatically adjust the row height to fit the text in the cell(s) you selected. If you want to adjust the column width as well, you can use the "Autofit Column Width" option in the same menu.

Note that this will only work if the cells are set to "Wrap Text". If you have a lot of cells to format, you can select the entire worksheet by clicking the box in the upper left corner of the worksheet (above row 1 and to the left of column A), and then following the steps above.