.single-product .woocommerce-variation-add-to-cart {
    display: flex;
    align-items: center;
}

.single-product .woocommerce-variation-add-to-cart .x-length-label,
.single-product .woocommerce-variation-add-to-cart .m-label {
    display: inline;
    margin: 0 5px;
}