How to get custom field image url of specific size

The get_post_meta() function can help to get the meta field but will not retrieve different size.

Assuming _pf_photo1 embed the attachment id, you can do something like that:

// Note the "_id" suffix
$attachment_id = get_post_meta($post->ID, '_pf_photo1_id', true);

Last parameter for this function can not be ‘medium’,

Now,you can use $attachment_id with different function depending on what you really want to get (url, img element…):

$attachment_element = wp_get_attachment_image( $attachment_id, 'medium' );
echo $attachment_element;

There is more ways to get details for attachment wp_get_attachment_url(), wp_get_attachment_image_src (that returns an array with url, width, height).

You will find more details to discover these functions here

Hope it helps !