@media only screen and (min-width : 992px) {
    .box-single-height .block-header {
        font-size: 1.5em;
    }
}
@media only screen and (min-width : 768px) and (max-width : 991px) {
    .box-single-height .block-header {
        font-size: 1.2em;
    }
}