how to change font size of MAIL, so I can read them

You can try this:

Go to “Settings” > “Control Center,” then find “Text Size” and tap the green plus sign. “Text Size” will now be part of the control center.

• Tap the Text Size icon (the one with the small and large A’s).
• At the bottom of the screen, you’ll see a toggle on the left for that specific app (In this case, you will see “mail only”, and the right side will say “All Apps.”) Move the toggle to the left.
• Once that’s done, use the main control in the center of the screen to set the text size for that app, and then tap anywhere on the screen to get back to the Control Center.

Hope this helps! Please mark as solved.

Mail>Preferences>Fonts & Colors>Message Font