I'm afraid you'll need to be more specific about your set-up and perhaps also your provider (Gmail ? Yahoo ? Opera ?).
If you're using a webmail interface, this is probably something you're going to have to do via your browser's settings (like setting a 'minimum font size,' having a 'zoom' add-on of some sort to increase the page fonts, or installing an add-on like Stylish which allows scripts to be installed which are site-specific and can do that job as well; I use all of these options at various places myself).
If, OTOH, you're using an offline email client (Outlook, Thunderbird, etc.), there should be some setting therein to allow you to increase the font size on incoming messages.