Paul,Depends on the receiver's email app. See attachments. Web browsers have similar facility (often CTRL + or CTRL -). If your email app sends emails in HTML format, there is a facility to set the font size at your end; the receiver can still over-ride the setting at his end.
Attachment:
email-font-big.jpg
Description: application/applefile
Attachment:
email-font-small.jpg
Description: application/applefile
