If you feel the 5.5-inch display on the iPhone 6 plus is too big to use with just one hand, Japan's Thanko Inc. has a solution for you.

The Tokyo-based gadget maker's "Yubi Nobiiru," available for ¥1,480 ($13.63), is a thumb-shaped stylus to be worn on the finger while using large-sized smartphones. It adds about 15 millimeters to...