I set my test box to 15,728,640,000 (15+ GB), set the MTA to Accept and Hold so I could validate the effective size of the e-mail and rule out downstream MTA issues.
I sent a
50MB attachment that Thunderbird says expanded to 77 MB (mime encoding overhead).
400 MB attachment that Thunderbird says expaneded to 558 MB (mime overhead again).
SBG shows these as
08/06/2010 14:35:15 79240911
08/06/2010 14:48:50 571466672
So the limit is larger than 400 MB / 571 MB accounting for mime overhead
Note: you need to make sure the next-hop MTA is also is correctly configured or SBG will bounce the message back during delivery phase.