I want to say size 14 but build matters a lot too imo.
I'm a size 15/16 (jeans/dress), 5'5'', 180-ish lbs, 44H-34-42 and consider myself a small(er)-BBW but a BBW nonetheless and wear the term quite proudly. I've also used Rubenesque, Zatfig, Voluptuous, Fluffy and Pleasantly Plump in reference to myself as well....I think they're all delightfully accurate too.
I've seen ladies with roughly same numbers as mine (weight, measurement, sometimes height etc) say they were NOT BBW's so.....some BBW's can't even agree on what is a BBW.
A "huge" BBW would be an SSBBW
http://www.urbandictionary.com/define.php?term=SSBBW