JavaScript

<<script type="text/javascript">
$.fn.textfill = function(options) {
    var fontSize = options.maxFontPixels;
    var ourText = $('span:visible:first', this);
    var maxHeight = $(this).height();
    var maxWidth = $(this).width();
    var textHeight;
    var textWidth;
    do {
            ourText.css('font-size', fontSize);
            textHeight = ourText.height();
            textWidth = ourText.width();
            fontSize = fontSize - 1;
    } while (textHeight > maxHeight || textWidth > maxWidth && fontSize > 3);
    return this;
}

$('div#my_text_box').textfill({ maxFontPixels: 100 });
<</script>

CSS

#my_text_box {
    border: 1px solid red;
    height: 50px;
    width: 70px;
}

HTML

<div id="my_text_box">
    <span>Hello World</span>
</div>