Try this, go to Internet Explorer and select PAGE from the Command Bar. If command bar not showing then right click near the address bar and select Command bar. Select TEXT SIZE from the dropdown menu and select LARGER or LARGEST to see how an email prints.
Right click on Control Panel and choose devices, Printers, there you will find the properties to change your default printer characteristics, font size and type, double sided printing etc, but not having a Deskjet I'm not sure of the layout but Control Panel ought to do it, or direct to Devices and printers will also get you there.
While you are there see if you can activate a popup to appear each time you hit the print button.
If you are using Windows Live Mail then open it and click on the first tab to the left of Home, scroll down to Options and then left click on Mail, then click on the Read tab and finally select Fonts and from that menu select Larger or Largest and then OK
Bob Exeter I think we may both be wrong, your suggestion is for outgoing mail and sets the default for emails that you send while mine is for reading incoming mail - but maybe neither has any effect on printing.