I put the define directive into wp-config.php file (into my test site) it took the site off, so I assume I had it in the wrong postion or line.
I copied your line excactly define(‘BPFB_OEMBED_WIDTH’, 250, true);).
If you could point me where to place it in wp-config.php also was the copy of the code, correct, that is, the likes of the full stop at the end of the code.(I tried it with and without the, what I call, the full stop)