Change Text Size commands Monterey help

Using the Command + or the Command - in Mail will give the appearance of the Text being Enlarge or Shrink but in fact the Actually Font Size within the e-mail has not changed

One can achieve the same Appearance of Enlarge via the Accessibility Feature is System Perferences

There is a text size selector in the Mail compose toolbar that can adjust your text size. Even without that amenity, you can select some existing text in the compose body, and then your keyboard shortcuts to increase or decrease text size will also work. They don't without some text selected beforehand though.

With a given Mail message selected, click in the message window and when you press cmd + or cmd -, the message text will change its size accordingly.