Hello,

Please add this code to the Theme Settings > Custom CSS > Global:

If your label is round use this code:

body .labels-rounded .out-of-stock {
    max-width: 100px;
    height: 70px;
}

If your label is rectangular, use this code:

body .product-label.out-of-stock {
    max-width: 100px;
}

Best Regards