Unfortunately, while conditional formatting in Excel allows you to format cells based on various criteria like font color, bold, or underline, it currently cannot directly change the font size or style based on conditions.

Here are some alternative approaches you can consider:

1. Manual Formatting: If you have a limited number of conditions, you can manually format the cells based on the font size or style you need. This might be suitable for small datasets.

2. Nested IF Statements (Advanced): For more complex scenarios, you can use nested IF statements within a custom formula for conditional formatting. However, this approach requires familiarity with Excel formulas and can be quite intricate.

3. Macros (Advanced): VBA macros offer the most flexibility, allowing you to write code that changes font size or style based on specific conditions. This is a powerful option but requires knowledge of VBA programming.

4. Alternative Formatting (Workaround): If the goal is to visually highlight data based on size or importance, you can explore alternative conditional formatting options like changing cell fill color or applying borders based on the criteria.