Your code works if you actually echo out the result:
<?php
$description = 'Large Flower';
$description = preg_replace("(Large)", " ", $description);
echo $description;
?>
You can use an array of regexps to process all of them:
<?php
$description = 'Large Flower
Medium Ivy
Small Tree';
$search = array(
'#\bsmall\b\s*#i',
'#\bmedium\b\s*#i',
'#\blarge\b\s*#i'
);
$description = preg_replace($search, "", $description);
echo $description;
?>