Your Mac has the abilty to zoom in on anything on the screen for easier reading, but you have to learn and practice a keyboard shortcut to make it useful. This is the only workaround I can think of for the inability to write emails with larger text for easy reading.
How to Set it Up
First go to System Preferences from the Apple Menu
Choose the Accessibilty Preference
Once in Accessibility Go To Zoom
Using Zoom with the Keyboard Shortcuts
With the checkbox ticked, your mac will now allow you to zoom the display by pressing down the "option-command" and tapping "8".
When the box above is refering to option-command "=", it's actually refering to the plus key "+".
Therefore, holding down option-command and pressing "+" or "–" will allow you to get to the right zoom level.
Finally, pressing option-command-8 can be used to switch zoom on and off to your preset.