#mapdiv {
    height: 750px;
}

@media only screen and (max-width: 768px) {
    /* For mobile phones: */
    #mapdiv {
        height: 1500px;
    }
}

@media only screen and (max-device-width: 768px) {
    /* For mobile phones: */
    #mapdiv {
        height: 1500px;
    }
}