A study shows us that a person is using a keyboard's size over 15 inches for years, he/she likely could get a long term health issue like scoliosis and spine pain. When we are using a standard size keyboard(~22 inches), our right shoulder will be always lower than our left shoulder because the space between mouse and the keys is too wide to make a balance to our shoulder. It causes a mussel disorder (or mussel adaption) when it is keeping at same tension for days and months. To avoid this health issue, we should use a keyboard size under 15 inch. It provides a suitable and comfortable space for our arms.