I have increased the max receive size for Exchange 2010 but it is still rejecting e-mail that is below the 20MB size limit, but over 10MB. I turned on protocol logging and found this.
2012-10-24T16:08:13.413Z,EXCHANGE\Domain,08CF8011F61E4312,41,10.0.1.13:25,208.65.144.245:13291,*,,AccumulatedMessageSize: 20987902 > MessageSizeLimit: 20971520
Which is understandable, except that the actual attachment is only 16MB. I then tried to send a 18MB attachment and it reports the exact same error with the exact same byte size. I don't understand why Exchange thinks that these attachments are all 20987902.