How to change font size for the messages, click the File tab and then select the Options from the available list. Now, open the Mail option and select the Stationery and Fonts options. Next, under the New mail messages or Replying or forwarding messages, select font. Once the Font dialogue box opens, select the font, style, color, that you want, and you can see the changes on the screen once you complete the settings. Once the settings are done, check it and then click the OK button to apply these settings.