I was thinking of a minimum piece count of at least 5,000 (kind of like Canada).
I think I'll see if I can get the figures on average mailing size at our BMEU.
I'll get it for all mailers except mailing houses since their volumes would skew the figures so far up as to make the data meaningless. Doesn't take too many 500,000 piece mailings to mess up averages for all mailers.