A cover letter should be written in a font size of 12 points so it’s easy for the employer to read. But you can reduce your cover letter’s font size to a minimum of 10 points to save space on your cover letter without losing too much readability.
Also, note that size varies between font types. So test the font you choose to see to find the ideal size for your letter. And if you’re unsure about the size you’ve picked, ask a friend to take a look and give you feedback.
Additionally, you should use an extra-large font (~30 points) for your name at the top of your cover letter to help the employer remember your application.