OS X Server includes a very robust mail server, that while not widely used by the IT community, uses tried-and-true open source software (dovecot and postfix) for POP, IMAP, and SMTP services. However, the Server app interface does leave many configurable options out of the user interface, and one important one is the maximum e-mail attachment size for your message. So, if you need to change it, you need to know the size in bytes. There are many megabyte to byte calculators on the internet.
First, find out the current maximum attachment size.
postconf | grep message_size_limit
This should return 10 MB in bytes, which is
message_size_limit = 10240000
Use postconf to change this. In this case, I am changing it to 50 MB
postconf -e message_size_limit=52428800
And finally, reload postfix
sudo postfix reload