Could an eye being smaller than the other be made even at an opticians?

Yes, with difficulty: A nearsighted glasses lens makes an eye look smaller, and a farsighted lens makes the eye look larger. Of course, that probably will not be the prescription that your eye needs. Contact lenses would have to be also worn to counteract the blurring caused by the glasses lenses. It would take some trial and error to find the exact power of lenses required to make the eyes look equal in size.