Samsung
Resize the Samsung Keyboard auf Samsung
The Samsung Keyboard ships in a fixed layout by default, which works fine on smaller screens but gets uncomfortable on larger phones. A size adjustment lets you shrink the keyboard and move it toward one side of the display, so your thumb reaches every key without stretching across the full width.
Where the setting lives
The option sits inside the keyboard settings, not in the general display settings, which is why many users never find it. Open Settings ➔ General management ➔ Samsung Keyboard settings ➔ Size and transparency.
Inside this screen, drag the borders around the keyboard preview to change its height and width. Dragging from the sides moves the keyboard toward the left or right, which is the actual one-handed mode. Confirm with Done to apply the new size immediately.
If the keyboard still feels off after resizing, check whether a third-party keyboard app is installed and active instead of the Samsung Keyboard. Third-party keyboards use their own size settings and ignore this menu entirely. The adjustment described here only affects the Samsung Keyboard on Samsung devices running Android with Samsung One UI.