As of now there is no direct way to conditionally set content_width.But we can do this using template_redirect hook and changing the global content_width.This is done in Twenty Fourteen Theme.Following is the code.
/**
* Adjust content_width value for image attachment template.
*
* @since Twenty Fourteen 1.0
*/
function twentyfourteen_content_width() {
if ( is_attachment() && wp_attachment_is_image() ) {
$GLOBALS['content_width'] = 810;
}
}
add_action( 'template_redirect', 'twentyfourteen_content_width' );