how to set postfix mailbox size limit and message size limit

By default, attachment size that can be sent by postfix is 10MB ~ 10240000 byte. How did I know it? I looked in log file (for my system it is in /var/log/mail/errors. For other system, the file to look is/var/log/maillog). The line looked like this:

Feb 26 16:30:53 webmail postfix/sendmail[30775]: fatal: me@mymailserver.org(74): Message file too big

Solution

Open /etc/postfix/main.cf with a text editor of choice and find message_size_limit directive and change accordingly. If it is not there, add the directive like this:

message_size_limit = 20480000

This sets limit to 20MB.

Some other parameters you need to change are in file php.ini which is usually located in dir /etc. Set their parameters as above or higher values as below:
post_max_size = 20M
upload_max_filesize = 20M

reload or restart postfix when you're done: 

service postfix reload 

or 

service postfix restart

There's also mailbox_size_limit directive. You need to change this if SM can not open mailbox sized more than 10 MB.