I've got Zimbra Version: 8.0.3_GA_5664.RHEL6_64_20130305090204 CentOS6_64 FOSS edition.

I've tried following the instructions in the wiki, but nothing I do seems to increase the maximum message size.

postconf message_size_limit
message_size_limit = 10240000
postconf | grep message_size_limit
message_size_limit = 10240000
zmprov modifyConfig zimbraMtaMaxMessageSize 20480000
postfix reload
postfix-script: refreshing the Postfix mail system
postconf message_size_limit
message_size_limit = 10240000

I had this problem on the old Zimbra install 7.xx but I cannot for the life of me remember how I fixed it.