Go to the "Tools" menu and select "Options." Click the "Mail Formats" tab and then click "Stationary and Fonts."
Look for the "Display Font" heading on the "Personal Stationary" tab and increase the size the size of the font by selecting a larger number. This will increase the text size for all plain text incoming email. However, it will not increase the text size of incoming HTML emails.
Set the option to read all incoming email as plain text if it is necessary to increase the text size of all incoming emails. To do this, go to "Tools", then click on "Options," then look for the button that says "Email Options" and click "Read all Standard Mail as Plain Text."