I recently had a AIX box send a 1.5 GB Email to our MS Exchange Email system, which brought Exchange to a screeching halt. Our Exchange admin was understandably unimpressed. So after a few seconds of research, I found sendmail has a setting to limit the maximum message size. Put this in your sendmail.cf file and restart sendmail:
O MaxMessageSize=50000000
That's in bytes, so that should be 50MB.