If you are using Plain Text view, the Windows* system default font is used for your message text. When the recipient opens the message, the recipient's Windows system default font is used. In the Mail From window, the recipient can click Edit > Font > Font, then select a larger size to make reading easier if necessary.
If you are using the HTML view, see "Changing the Default Font in the HTML View". The recipient views the message in the same font and size as you composed the message.