#textBoxContainer {
width:100%;
height:30%;
position:fixed;
z-index:99;
bottom:0px;
left:0px;
background-color:#999;
border: 4px solid #000;
}

<div id="textBoxContainer"></div>

#textBoxContainer {
    height: 30%;
    position: fixed;
    z-index: 99;
    bottom: 0;
    left: 0;
    right: 0;
    background-color: #999;
    border: 4px solid #000;
}

-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
-ms-box-sizing: border-box;
box-sizing: border-box;