.grid-sizer,
.portfolio-item {
    width: 20%;
    height:200px;
    background-size:cover;
}
.portfolio-item.w2 {
    width: 40%;
    height:400px;
}

.port-item img {
    width: 100%;
    height:auto;
}

.port-item-row {
    margin-top: 30px;
}

.overlay {
    background:transparent;
    position:relative;
    width:100%;
    height:500px; /* your iframe height */
    top:500px;  /* your iframe height */
    margin-top:-500px;  /* your iframe height */
    z-index:999;
    cursor:pointer;
}