.main_clearfix{
    padding: .5rem 0;
}
#sb-search {
    display: inline-flex;
    align-items: center;
    position: relative;
}

/* иконка */
.sb-icon-search {
    cursor: pointer;
    width: 2rem; height: 2.6rem;
    margin-right: 0.5rem;
    display: inline-flex;
    align-items: center;
    justify-content: center;
}

/* ========== Инпут (закрытое состояние) ========== */
.sb-search-input {
    position: absolute;
    top: 0;
    right: 0.3rem;
    width: 0;
    padding: 0;
    border: none;
    font-size: 1rem;
    line-height: 1.5;
    box-sizing: border-box;
    opacity: 0;
    transition:
            width 0.3s ease,
            padding 0.3s ease,
            opacity 0.3s ease;
}

/* ========== Инпут (открытое состояние) ========== */
#site-search.sb-search-open .sb-search-input {
    width: 41rem;               /* ваша желаемая ширина */
    padding: 0.5rem 1rem;       /* внутренние отступы */
    border: 1px solid #ccc;
    border-radius: 0.25rem;
    background: #fff;
    opacity: 1;
    z-index: 10;
}
#site-search{
    display: inline-flex;
    align-items: center;
    position: relative;
}