@media only screen and (min-width: 100px) and (max-width: 640px) {

            div {

                width: 100px;

                height: 100px;

            }

        }

@media only screen and (min-width: 641px) and (max-width: 789px) {

            div {

                width: 200px;

                height: 200px;

            }

 }