A size adjustable footwear includes an upper, an insole, a midsole, an outer sole, and a size adjustment mechanism. The midsole includes a heel plate and a toe plate. The upper is perimetrically connected around the midsole. The insole is adjacently connected atop the midsole and internally positioned within the upper. The outer sole is perimetrically connected around the midsole as the midsole is positioned in between the insole and the outer sole. The heel plate and the toe plate are slidably engaged to the size adjustment mechanism. The size adjustment mechanism is operatively integrated in between the heel plate and the toe plate, wherein the size adjustment mechanism elastically expands and contracts the upper, the midsole, and the outer sole.