Reduce font-size of text in $target to fit into a single line if necessary.

Reduce font-size.js

var $target = $('#selector');
var target_width = $target.parent(':first').width();

var $span = $('<span></span>');
$span.css({
  'display': 'inline',
  'visibility': 'hidden',
  'font-size': parseInt($target.css('font-size'), 10),
  'font-weight': $target.css('font-weight'),
  'white-space': 'nowrap'
});
$span.text($target.text());
$span.appendTo('body');

while($span.width() > target_width && parseInt($span.css('font-size'), 10) >= 5) {
  $span.css('font-size', (parseInt($span.css('font-size'), 10) - 1) + 'px');
}

$target.css('font-size', $span.css('font-size'));

$span.remove();

Not 100% full-proof depending on block-level CSS, and requires a parent container around $target that we want to fit the text in.