JQuery :: Resizing Font-Size According To Amount Of Characters

I'm looking for a way to resize the 'font-size' of an element so it fits in it's container. What I currently have is this:

var maxWidth = 100;
var $width = $('.number span').width();
while ($width > maxWidth) {
$width.css({ 'font-size' : '-1px' });
alert("bigger");
}

I was trying to make it so while the width of the text is larger than the width of its container, 100px, the size would decrease by 1 in a loop until it's the correct size. Unfortunately, this yield's no results for me.