crop custom field image size

// get data form post meta
$dataMeta = get_post_meta( get_the_ID(), 'my-metafield', true );

// make sure your post meta isn't empty
if(!empty($dataMeta)) {

	// image url
	$image_url = $dataMeta;

	// retrive attachment ID
	$id = wplap_get_attachment_id_from_url($image_url);

	// retrieve the size of yur image that you want to set
	$image_thumb = wp_get_attachment_image_src($id, 'custom-thumb' );
 
	// display the image url with size
	echo $image_thumb[0].'<br />';

	// display as a image with size
	echo '<img src="'.$image_thumb[0].'" width="'.$image_thumb[1].'" height="'.$image_thumb[2].'" />';
}