One of the lesser trumpeted features that shipped with iOS 11 was the one handed keyboard. The linked post walks through the options.

Long story short, fire up a text or tweet, then, when the keyboard appears, press and hold the keyboard’s emoji icon, like so:

Tap your keyboard alignment of choice (left, right, center) and you’re good to go.

Note that the one handed keyboard is iPhone (and iPod Touch) only. Read the article for iPad options.