i need functions

hi
i need to a function to get new thumbnail in any size. for example 200×200
For example :

<?php
// get the first image attached to the current post
function aldenta_get_post_image($size = 'thumbnail') {
global $post;

$photos = get_children( array('post_parent' => $post->ID, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC', 'orderby' => 'menu_order ID') );

if ($photos) {
$photo = array_shift($photos);
return wp_get_attachment_image($photo->ID, $size);
}
else{
	return '<img src="'.get_bloginfo('template_url').'/default.jpg" border="0" />';
}
return false;
}
?>

echo thumb:
<?php echo aldenta_get_post_image();?>