Edit in JSFiddle

$(function(){
    var biggestHeight = "0";
    $(".parentBox *").each(function(){
        if ($(this).height() > biggestHeight ) {
            biggestHeight = $(this).height()
        }
    });
    $(".parentBox").height(biggestHeight);
})

<div class="parentBox">
     <div class="childBox">Box</div>
</div>

.parentBox{
     position:relative;
     width:100%;
     border:solid 1px #f00;
}
.childBox{
     position:absolute;
     height:100px;
     background-color:#0f0;
}