This problem has come up quite a few times and it usually seems to be related to the questioner using the wrong 'Print' option.
If you simply press Ctrl and P together (or go to File > Print) you're telling your computer to print the whole of the web page, rather than just the email. That results in very small type. Instead you need to use the 'Print' option at the top right of the email:
https://support.google.com/mail/answer/8255?hl=en