Opening links from mail app
When I click on a link in an email, my iPad opens a new window that is only about 1/4 the width of my screen. I used to see a full-size new window.
iPad Pro, iOS 12
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You have opened a Slide Over - which will occur if you touch-and-hold a link (i.e., a “long” touch) instead of a simple “tap”.
To transform a Slide Over view, to a Split Screen, use the grey drag-bar at the top of the Slide Over Window; drag downwards and to towards the side of the screen. As the view changes, lift your finger. You can also change a Slide Over to a Full Screen view. This time, using the same drag-bar, drag to the top-centre of the screen, then release.
You may find these overview Apple video guides to be helpful:
After viewing, you’ll be ready for the additional Multitasking Guide:
Use Multitasking on your iPad - Apple Support
The iPad User Guide for iPadOS is also a rich source of information. Here are links to both the downloadable and online versions of the Guide:
iPad User Guide on Apple Books
iPad User Guide - Apple Support
I hope this explanation and additional information is helpful in resolving any difficulties you may be having with Multitasking features of your iPad.