There are a few different ways that you can get your keyboard back to full size. The first way is to simply adjust the settings in the Keyboard section of your device's Settings app. To do this, open the Settings app and tap on Keyboard. Then, tap on the Keyboard Size option and drag the slider to the right to increase the size of your keyboard.
Another way to enlarge your keyboard is to enable the Accessibility feature called Display Zoom. This will make everything on your screen, including your keyboard, appear larger. To turn on Display Zoom, go to Settings > Accessibility > Display Zoom and select the Zoomed option.
If neither of these methods work for you, it's possible that your keyboard has been set to a smaller size by default. In this case, you'll need to change the default keyboard setting in order to get your keyboard back to full size. To do this, go to Settings > General > Keyboard > Keyboards and tap on Edit. From here, delete any smaller-sized keyboards that may be listed and then add a new keyboard by tapping on + Add New Keyboard. Be sure to select a full-sized keyboard from the list of options so that you don't run into this problem again in the future.