You can use this method to break your text so that it'll fit a certain $maxWidth.
<?php
protected function _fitText($text, $maxWidth)
{
    $im = new Imagick();
    $im->newImage($this->_width, $this->_height, "none");
    $lines = explode(PHP_EOL, trim($text));
    $DEBUG_LOOP = 0;
    for ($k = 0; $k < count($lines); ++$k) {
        do {
            $drawText = new ImagickDraw();
            $metrics  = $im->queryFontMetrics($drawText, $lines[$k]);
            $fits     = $metrics["textWidth"] <= $maxWidth;
            if ($fits) {
                break;
            }
            $pos = mb_strrpos($lines[$k], " ");
            if ($pos === false) {
                throw new RuntimeException("can not make it fit");
            }
            if (!isset($lines[$k + 1])) {
                $lines[$k + 1] = null;
            }
            $lines[$k + 1] = trim(mb_substr($lines[$k], $pos + 1) . " " . $lines[$k + 1]);
            $lines[$k]     = trim(mb_substr($lines[$k], 0, $pos));
            if (++$DEBUG_LOOP >= 200) {
                throw new RuntimeException("infinite loop");
            }
        } while (!$fits);
    }
    $text     = implode(PHP_EOL, $lines);
    $drawText = new ImagickDraw();
    $metrics  = $im->queryFontMetrics($drawText, $text);
    $metrics["text"] = $text;
    assert('$metrics["textWidth"] <= $maxWidth');
    return $metrics;
}
?>