My wife and I have identical iPads running OS 16.4.1 and, for a while now, links in email are displayed as a narrow band about 1/3 of the screen instead of full screen. I found the following Apple link which appears to have resolved the issue on my iPad (using the "Touch and Hold option) but not my wife's.

How to open a center window on your iPad
Open an app that supports center window, such as Mail or Notes.
Touch and hold an individual email, note, or other item from the app's list.
Tap Open in New Window in the menu that appears.
To close the center window, tap Done or Close. You can also tap the Multitasking button, then tap one of the buttons to change the center window to full screen, Split View, or Slide Over.

The second option "You can also tap the Multitasking button, then tap one of the buttons to change the center window to full screen, Split View, or Slide Over" does not appear with the multitasking button, on either of our iPads. I did a reset of my wife's iPad but that didn't help.

I suspect the instructions and the functionality are stiil a work in process, but apparently the "muti-tasking button" is the elipsis at the top of the window. I couldn't get this all in one image but start here, and the "big reveal" is here.

That navigational advice appears to have done it. I was looking at the three dots in the circle to the top left rather than the hard-to-notice three dots in the top middle. Thanks.

Ooops. Top right, not top left. Hmmmm. Maybe that explains things even more.

You did mean to say the OTHER left didn't you?