Here is the jQuery code :
$('input').each(function(){
var value = $(this).val();
var size = value.length;
// playing with the size attribute
//$(this).attr('size',size);
// playing css width
size = size*2; // average width of a char
$(this).css('width',size*3);
});
http://jsfiddle.net/bzBdX/7/