Ipad mail how to resize the mail message on the right hand side of the screen

I note from your screenshot that you have enabled Stage Manager. As a diagnostic exercise, try disabling Stage Manager - and then repeat the forced-restart as previously described.

Settings > Multitasking & Gestures > [Multitasking] - select either Split View and Slide Over or OFF

I have experienced this behaviour on one occasion. Try a simple forced-restart of your iPad - as this will usually clear an isolated software or performance issue:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

https://discussions.apple.com/content/attachment/34445a41-a451-4535-b45a-d70b3a374578

I'm delighted to learn that the problem has been identified and easily resolved. Stage Manger is arguably of limited usefulness when using an iPad without an external monitor.

Having now corrected the problem, should you choose or need to do so, you can try re-enabling Stage Manager. With the issue now cleared, you will likely find that the problem does not return - but if it does, you know how to clear it.