Display post image in a size defined by add_image_size()

Try using wp_get_attachment_image() (Codex ref)?

echo wp_get_attachment_image( $attachmentid, $imagesize );

Where $attachmentid is the attachment ID, and $imagesize is the name of the image size in the add_image_size() call.

EDIT

For a custom-field stored image SRC URL, I’m not finding any way to make use of intermediate image sizes defined via add_image_size() (or even the default image sizes). Pretty much all of the related functions eventually require an attachment ID.