First read out the value of your current message_size_limit with the command postconf | grep message_size_limit
If needed, you can change the value of message_size_limit by editing the postfix configuration file /etc/postfix/main.cf.
#Accept maximum 10MB:
message_size_limit = 10240000
message_size_limit = 0
After changing your postfix configuration file, you reload postfix:
If you have a webmail client installed on your server and you want to send bigger attachments with it, also change the values of post_max_size & upload_max_filesize in your php.ini file.
; Maximum size of POST data that PHP will accept.
post_max_size = 10M
; Maximum allowed size for uploaded files.
upload_max_filesize = 10M
After changing your php.ini file, reload apache with: