.main-slider .bx-viewport {
    height: 600px !important;
}

    @media only screen and (max-width: 767px) {
        .main-slider .bx-viewport {
            height: 500px !important;
        }
    }
    @media only screen and (max-width: 500px) {
        .main-slider .bx-viewport {
            height: 400px !important;
        }
    }