Partial screen in mail

You perhaps have a Slide-Over view enabled, this being a feature of iPadOS.

The easiest way to switch from Slide Over to a full-screen window is to touch-and-hold the small drag-bar at the top of the Slide Over Window, drag to the top-centre of the screen - then release; this window will become full screen.

Here is an Apple video that demonstrates use of Slide Over:

https://youtu.be/ITzy5J3j5Is

... and the corresponding page from the online version of the iPad User Guide for iPadOS:

https://support.apple.com/en-gb/guide/ipad/ipadfe7c65e9/ipados

... and a further help page that describes many associated features of iPad multi-tasking:

https://support.apple.com/en-gb/HT207582

I hope this information os helpful is resolving your immediate question - and in providing useful references to significant functionality. 

See this:--> Take a screenshot on your iPad - Apple Support

then post it using the "Image" icon at the bottom:

Regards

Giulio

Are you using Apple Mail app? Could you please post us a screenshot showing that "thin" screen?

Thanks

Giulio