Full-width / half-width switches automatically
The “Smart Full-width Space” function can be switched on / off from the “Keyboard” in the “Settings” app. Normally it is on, but if you always want to enter half-width characters, turn off the “Smart Full-width Space” switch.
▲ Half-width space (left) for English keyboard, full-width space for Japanese keyboard (right)
By the way, with the QWERTY keyboard (Romaji input), you can switch between full-width / half-width by pressing the “↑” key. “Half-width space” is displayed when half-width characters are entered.
▲ Drag on the keyboard while holding down the “Blank” key