For Mail I use an app, HTML Mail Editor as my starting point. (It's only 99¢.) I even have it in my Dock to start my emails.
With it you can create simple little templates specifying your font face, size and color for quick, large typeface emails, or you can edit your outgoing email directly in the App for more complete control. It's pretty easy.
Example template "Helvetica 16" :
<font size="16" face="Helvetica">REPLACE
</font>
Once that is created in the HTML Mail Editor, there's a Send button. I just click send and it's placed in an outgoing email, I select and type over "REPLACE" within Mail and the email is in a bigger font. Of course you can change the font and size number to make whatever simple templates you want. I do it this way so the templates are always ready. If I edit in the program, I mess up the template.
See http://iosfonts.com for the font options. This opens up a lot more fonts and control than you normally have.
ivan
( see also
https://discussions.apple.com/thread/4852686?answerId=21457988022#21457988022
https://discussions.apple.com/thread/3402998?answerId=19568997022#19568997022
)