Changing the size of the text in your email is also called changing the size of the font. Yahoo email is used by a number of people including some that do not realize it. If you have an email account with AT&T (att.net) or one of the various former telephone companies that AT&T has taken over you are probably using Yahoo for your email. This includes Bellsouth.net
To set print larger you can do one of two ways.
First way is to click on the gear in the upper right corner of the screen, then click settings in the menu, then choose writing email and then change the default size to large or size you want. This will change the font/text in all future email emails.
The second way will just change the text/font in the current message you are writing. Click on the Tt button at bottom, and then change to the size you want in the right column.
Thanks Carl for the question