Make sure the define is near the top of the wp-config.php file. If it is near the bottom, below the include/require line, then it will not do anything.
Try doubling the value. The value below will download 1.6Mb of the media file to get the meta information.
define('POWERPRESS_DOWNLOAD_BYTE_LIMIT', 1600000);
Also, reply with the URL to the media file in question so we can test.