SOURCE

<div class="box">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
</div>

<style>
    *{padding:0;margin:0;box-sizing: border-box;}
    div{
        min-width: 10px;
        min-height: 10px;
        background: rgba(0,0,0,.1);
        border: 1px solid #f00;
    }
    .box{
        display: flex;
        flex-wrap: wrap;
        flex-direction: column;
        height: 100vh;
        align-items: center;
    }
    .item{
        width: 50px;
    }
    .item:nth-child(1){
        height: 100%;
    }
    .item:nth-child(2),.item:nth-child(3),.item:nth-child(4){
        height: 30%;
        flex: auto;
    }
    .item:nth-child(3){
        margin-left: 100px; 
    }
    .item:nth-child(5),.item:nth-child(6){
        height: 50%;
    }
</style>