Change reading/writing fontsize in Mail?

Yes, what ever you set in Preferences>Fonts and Colours is what is seen at the other end, and, you've guessed it, whatever the sender sets in his Fonts and Colours is what you see when you read his email. You can't change what arrives in your inbox.

 

I was wondering about this and checked if 2 options were possible, and they both worked on my iMac (using 10.9 Mavericks, don't know if they'd work in El Capitan or Yosemite).

First, on received mail: I highlight the text body and then hold down the Command key and tap the + key several times, it enlarges the text just fine.

Second, to enlarge text while writing an email --but keeping the original font size for the recipient: hold down the Control key and scroll your mouse to zoom in. You can type your email text and when finished, press the Escape key, or Control key & scroll the mouse in the reverse direction until back to original screen view.

Not terribly elegant, but functional.