If you can't adjust the font directly in the email, every printer I have seen offers you an option to print larger than the what the default is, you just need to go to the options and change it when you print.
How your email is printed depends a lot of your email client - stand-alone (eg Mail app on Windows 8/10, Outlook express / Outlook), or Web-based (GMail, Hotmail. Yahoo etc)
If you can't adjust the font directly in the email, every printer I have seen offers you an option to print larger than the what the default is, you just need to go to the options and change it when you print.