Size of font, when reading (not sending)

When I view Apple Mail, in editing mode, there is the font size that I view.

Then there is the font size that actually gets transmitted.

How can I make the view font size bigger please?

Unlike Word/pages/Excel which all have 100%/150%/200% view options, this has none. And in Safari you just CMD and + to make the view size bigger. How come it works on this page here but not in ordinary Mail?!

For eyesight reasons I struggle to read the small font, and can't get the big screen closer.

Apple Genius Bar folk say there isn't a solution, other than make it font size 20 to edit, then change it to font size 12 to send. That seems lame.

Anyone any ideas please?