.parent {
    width: 100%;
    height: 100%;
    display: table;
    text-align: center;
}


.parent > .child {
    display: table-cell;
    vertical-align: middle;
    font-size: 100px;
}

@media (max-width: 535px), (max-height: 275px) {
    .child {
        font-size: 50px !important;
    }
}

@media (max-width: 255px), (max-height: 125px) {
    .child {
        font-size: 25px !important;
    }
}